Sideways has the highest hp daily driven SC car that I know of on S2ki. Around 450whp IIRC
http://www.s2ki.com/forums/index.php...=215021&st=100
That's the thread following what has been done to his car.
The extra gain isn't from exhaust and other bolt-ons. It's from running more and more boost and building the rest of the engine to not blow up.
I can't be sure, but I think he might be at the limit of what that blower can do. With a turbo, you can go higher, 800hp would be possible, for a dyno run or maybe a 1/4 mile... but not for running around picking up groceries week after week.