Standard B7 RS4s have been measured (using GPS) doing 168mph on German autobahns (so the 155mph limit isn't accurate), unlimited ones AFAIK only make it to just over 180mph - so it's big bucks for a small (and rarely usable) increase.
The chip has made throttle response up to 5k more responsive always find the high revving not so good under 5k. It's the first v8 I have owned that you have to rev the knackers off it to make it move. Now the problem is better but still prefer a lower revving v8. Still looking seriously into supercharger route.
Had 165mph (on the clock) out of mine standard (abroad) and it still felt like it had some guts left. Felt planted too.
Supercharger (Danny RS4 on 246.com) I believe is running around 565bhp with a PES charger and silly torque. I would imagine well over £10k for that setup.
I've been quoted £4200.00 + vat for custom remap, Milltek valved non res, manifold polished and ported, cold air feed, & filters which will get me up to 465bhp ish. I'm currently just under 400bhp standard. You're right - a lot of money for the gains.
Spend £5k on a B5 RS4 and I'd love to see those figures.