Just to clarify:
The frictional force is a function of the area of the pad, not the swept area of disc. (Basically the product of the effective fluid pressure acting over the pad area). The fact that the swept area is greater (on a larger disc) only affects the heat dissipation. It's the lever arm effect (ie bigger radius) that increases the stopping force as per Rupert and my parallel posts above.Pad contact area is increased per wheel revolution.
As the discs are dissipating the heat better they run cooler. This reduces wear, reduces distortion and cracking, and prevents the pads from burning as James seems to fear. In other words, moving the centre of action of the brakes outwards will keep the pads cooler.
As for the effect of putting bigger pads/calipers on - if they are basically covering the same arc, but simply extend nearer to the centre of the wheel, the effects will be very little - minimum effect on braking torque, and very small negative effect on heat dissipation.
If you want to improve braking you either have to put caliper/pads on which embrace a greater arc of the wheel (but this risks more heat build up as the ventilation is restricted), or move the braking point radially outwards with bigger discs - which is precisely what fitting the C6 discs achieved. Main downside is increase in unsprung mass - more loads into our tired DRC's (or new coil-overs if you prefer)!!
And finally a question - Matt1979 - why are you looking to swap back? Just intrigued!
Dave