Well you can - via the Hamman kits. However it's not cheap - full kit for the top power is something like £20k. They you have to get it fitted.
DMS will remap BMW engines - and they do a good job by all accounts. I know of an S1 putting out 320bhp via a remap and a sports exhaust.
RS4 is turbocharged. No room in Aero engine bay for super / turbocharger though. (Though an old school style US supercharger poking out the bonnet hotrod styleee on a Series 1 would look cool. Just won't be able to see where you are going though...
I spent agest looking at tuning the S1, in the end the cheaper option for more performance is to buy a series II,III,IV - as as well as more power you get a whole host of other goodies for free..
The RS4 4.2 V8 engine is in fact normally aspirated.
