Importance of Load Index (tyres)

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As for the load index, I'm still not sure what to do. The R888 are what I want, but they are 93Y instead of 96Y... Arghhh. What are the risks, if any? Bearing in mind there must be a fairly huge safety margin if the RS6 is also running 96Y.

Yes... and no...
Need to consider that whilst the RS6 is heavier than an RS4 it is also has a longer wheelbase.
The only factors that influence how much load gets transferred during braking are:
1) mass (duh)
2) centre of gravity height
3) wheelbase.

(I'm not counting brake pressure, tyre grip etc... Assuming the same deceleration)

Everything else such as suspension characteristics simply define how fast the transfer happens in terms of the tyres point of view (soft suspension means it happens gradually and slowly, hard suspension much faster, no suspension like a go kart is near instant)

I've no idea of the stats of the RS6 but it's things like this to consider... it may be that the 96y is overspec'd for the rs4 (and might simply be the appropriate tyre used due to commercial agreements with Pirelli)... Hard to say.

You then have the other issue that if the r888's are up to temperature then braking forces will be higher and load transfer even more... They may take this into account hence the reason load indexes are generally lower on track tyres... no idea.

I have my own opinions on track tyres for non-track cars and I'd say get the nastiest cheapest set of to-spec road tyres you can and enjoy them. But it really depends on what you want out of the day and how experienced you are.

[Shoppinit]

I'd say get the nastiest cheapest set of to-spec road tyres you can and enjoy them.

I'm not sure what you mean by this. Can you elaborate?

But it really depends on what you want out of the day and how experienced you are.

I hear what you are saying, but I find that I am finding the limits of the PSS far too quickly on the track - more in cornering than in braking, TBH. I don't think I'm particularly skilled, but I feel that the car would benefit from more grip before I find the limits of what a road car can do. By the way, I'm talking about the M3 here. No problem finding appropriate track day tyres.

Do you think I would notice any appreciable improvement with the AD08 or Corsa, even though they are more road oriented? Again, talking about the M3 here.

The RS4 was 1 or 2 secs per lap slower with the PSS over the PZero... food for thought.

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Really depends what your objectives are on a track day.
Can be anything like-
- fun
- learn
- go fast

Ironically there's not as much overlap as you'd think.
To go ultimately as fast as you can is very draining for most people. To hold the car on the absolute limit is hard going.
This can not be as much fun as other things and you are generally so focused on the detail that you don't actually learn anything.

Fun is just not taking anything too serious and just having a play with no ultimate goal. You don't go fast and you don't learn huge amounts.

Learning is a nice balance as you aim to go a little bit faster bit not so much you have tunnel vision.

For the last two you really want the crappiest slippery tyres you can get that you don't mind abusing.
The bonus is that the low grip really reduces the wear n tear on the car. It also makes things happen way more slowly and with a larger margin of error and more predictably.

Track tyres will (if you get the buggers up to temp) be ultimately faster but are a lot more temperamental. The optimum slip angle is much reduced and the difference between gripping and sliding happens much quicker.

Don't get me wrong, track tyres are fun but have their pros too.
(And don't forget track tyres absolutely don't work in the rain... To the point I've abandoned track days as its just not fun).

Probably I'd be asking in what conditions the tyres are losing grip to suggest softer stickier rubber may be in order?
As in is it high speed or low speed corners? Corner entry, mid point or exit?
Simply adding stickier tyres might work but by masking something else than actually fixing it iysim

[Shoppinit]

Definitely learning is the big priority. I'm not that competitive so I'm not interested in being faster than everyone else, but I can't help thinking that the tyres are holding me back.

Maybe going for some less track oriented tyres like the AD08 would be a good start then...

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What pressures do you run (relative to stock)?
Have you had any decent track instruction?

[Shoppinit]

On the M3 I have tried different pressures (mostly by lowering) and found that leaving them at stock pressure gave me the best results.

I've had some instruction which has helped a lot. I'll be having some more. I don't think my lack of skill is a factor in the grip issues I'm experiencing. Of course, it's still quite a heavy car so that's playing a large role, but I'm not quite ready to strip it out yet.

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That's a good sign - most people drop the pressures too much. A softer tyre acts like softer suspension in that ultimate grip is a little better (subject to not over deforming the tyre) and things happen slower and predictable... the problem is that it's just not fast (and wrecks the tyres). Properly inflated the tyres may feel like the car is on stilts and even a little nervous but that's generally what you want. The only real way to check how the tyre pressures are working for that day on that track with that car and your driving style is to get a tyre pyrometer and measure the temps in different places on the tyre surface after a few laps. You are looking for broadly a linear temperature from the outside to the inside (to take account of any negative camber). You know if you've under inflated as the middle doesn't get anywhere near as hot as the edges.

What is happening to the car dynamically when you feel the tyres have reached their limit?

[Shoppinit]

I think the speeds you get up to on the track are deceptive, too. Those nice big run offs and safe gravel traps and reassuringly wide track give you the impression that you are not going as fast as you are. You'd never push a car that hard on the road unless you were a) suicidally reckless or b) footlong.

That's probably why I'm finding the limits of the tyres so quickly.

Dynamically, what is happening when I most miss the grip is probably coming out of corners. I know it's a fault of mine to want to put the power on too soon (bloody quattro spoiled me) and I'm trying to rein that reflex back in, but even when I feel I'm doing things right, the tyres feel greasy like they are squirming around a bit too much and ready to break away.

It's more difficult, I think, to judge the effect of the tyres on the braking. The car actually brakes pretty well I find, but it would be nice to have a bit more ability to scrub off speed.

[sixspeed]

Dynamically, what is happening when I most miss the grip is probably coming out of corners. I know it's a fault of mine to want to put the power on too soon (bloody quattro spoiled me) and I'm trying to rein that reflex back in, but even when I feel I'm doing things right, the tyres feel greasy like they are squirming around a bit too much and ready to break away.

You've hit the nail on the head. You're trying to apply power too soon. Going up to a "stickier" tyre won't stop this happening, you'll just be going faster when you do.

I run Michein PSS on my Z4MC track car, and find them incredibly grippy, and the car is very well balanced. Ultimately I'm slower than if I had a full-on track/cup tyre, but oversteer at the rear, under power out of corners, is down to the driver's right foot being too keen (and to some degree, suspension setup).

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+1.
Also another possibility is that you have too much lock on for the power you are putting down.

The 'ideal' on a normal constant bend is to ip the apex and then progressively apply the power as lock comes off.
This isn't rocket science and everybody knows this. The issue is that with rwd any steering lock on the front wheels fundamentally reduced the amount of power the reads can apply (they almost act like a brake).
The art of the last 10% cows from being able to rotate the car during the corner but with the steering wheels almost straight ahead. So after the apex you need to apply throttle such that the car is rotating more from that than steering.
Bloody easier said than done though! But I you can do it you'll be able to go much faster without any changes.