My friend's 06 STI made 387whp on a dynopack and got about 17mpg mixed.
As far as the power level it was fast but it wasn't uncontrollable like some of the posts here say. Wasn't very hard to drive at all when I drove it. My 330whp Spec V was harder to drive with all the torque steer but even that wasn't that hard to drive.
Then again I've been to the strip many times and autocross my car a decent amount.