I didn't fancy putting £10k into charging hence why I avoided the V8 S5 and came back when the coupe was re-shaped as a V6T. The V8 is still the more symphonic engine, and a glorious piece of technology, but likewise the V6T lump has it's fair share of aces up it's sleeve. I felt there was more noise that power in the V8, it didn't quite go like it sounded even though it was a properly quick. The linear delivery really masked the power after years of turbo cars.
There is so much going on under the bay I think you'd have to do major work to fit the relevant cooling system in to support turbo charging. I'm not convinced despite the compact dimensions it would all fit. Audi predominantly turbo charge all their range, so to decide to S/C the V6 rather than whack a couple of turbos on it says there may have been a few more challenges in the turbo route than the Eaton route.
With a S/C the V8 will be transformed, but only Cuppa will be able to answer that one as I've not been in one.