I echo other comments on the V8.. Nice sound but that's about it. They also seem to RR at well under their claimed output (probably why they are a lot slower than they claim to be).
Couple of quotes from MRC Tuning regards RS4's that have been in for work/heath checks..
"Health Check Run: 309ps 401nm. Fixed broken solenoids and leak: 362ps 429nm, MRC Stage 2 Remap and Filter: 391ps 460nm"
"Before Clean: 364ps 443nm. Cleaned, ported and MRC Stage2: 428ps 472nm"
"Stock car health check 387ps 437nm"
"371ps 412nm. Next: Carbon Clean, Ported Manifold, Precat Removal and Stage 2 MRC Remap."
The list goes on...
If you can stretch the pennies and want a S4/5 then I would head for the V6 SC'd lump..