Nick,
Not been in here for some time and I'm sorry to hear about your RS4..... at least you're OK!
With regards the new S3. In all honesty it's pretty damned good out of the box, way better than the 8L ever was.
However, since playing with my car I can recommend:
Remap - GIAC or REvo are the maps of choice - both giving good gains on stock hardware. The GIAC from rolling road days appears to be smoother and a little more driveable. Revo has more of a wallop when the turbo kicks in if that;s your thing. I went GIAC for mine and have had no issues with 314bhp and 310lbft

. Facelift models had some changes to the downpipe and some have had mis-fire issues after maps..... I think the map companies are working on a fix, but replacing the zorst cures it.
I'm hoping to add a Evoms CAI and custom Milltek (in discussion on this one at the moment) and with the GIAC map it'll adapt to about 350bhp or revo stage 2 at around 340bhp.
To get better torque you'd need the HPFP updating as the standard fuel pump struggles through the mid range.
Suspension - I've dropped mine on H&R springs (25mm drop), H&R ARB's and H&R Hub centric spacers 10mm at each corner. The ride is actually slightly suppler around town, but definately firmer at speed. Direction changes are far better, understeer is pretty much eliminated and the car feels far more alive. If you have the funds the H&R coilovers have been recommended too.
Brakes - standard items are fine, even for occassional track use. Uprated pads and braided hoses are all you'd need to do.
I'm actually getting the engine and gear box mounts updated tomorrow to improve response and the lower arm bushes uprated to improve steering response.
It's a great car and will have it for another 12 months then ponder what to change too.
I'm tempted in the econimic doom and gloom to capitalise and get a good price on an RS4 (V8), but a big part of me wants another S3!!!
HTH
J