Well Mark @ Tdi reckons approx 15-20bhp increase (its the hoses that do it :P )
If your not gona go the SuperCharger way then i highly recommend Unichip,, You can notice the increase in response and the smoothness in acceleration, cant say what extra mph its given me as i havent gone flat out (again ) allegedly
No engine mods on the IS will give you a greater top speed without either:
1.) Removing the rev limiter
2.) Changing the transmission ratios
3.) Changing the engine/transmission (like on Clawley's beast)
The s/c cars still top out at an indicated 141mph, the same as the n/a ones, the only thing you can alter is how fast it gets there!