100% agree with what RedRobin said, nobody will want to buy tuned cars (would you??)
Also, haven't you read Wolfbane's post, he has 330hp and 435Nm of torque, and that is ENOUGH already, wheels are spinning left and right in 3rd and 4th gear, imagine what it would be like with 380hp and 500+ torques - a total nuissance!
people wanting stage III for their E30 Golfs are either hard-core track drivers (dry track, of course), or completely deranged.
P.S. That kind of power would be nice for say S3 or TTS, but then you need new clutch at least. And I'd be worried about engine life, since it would be hard to sell a car hard-tuned like that anyway.