Yeah, I can't argue with your experience. Drivetrain would definitely be a mod but I wouldn't even mess with the STi motor, I would just go all out with a Cosworth built block (which technically does have an STi engine case and crankshaft), Cosworth Heads, Cosworth intake manifold, APS Twinscroll turbo set up...pretty much replicate the DynoComp STi (engine wise). And I have no idea who I would want to tune it...
http://www.airpowersystems.com/wrx/a...p/2006_sti.htm
But I won't ever go that far. I would rather have an Evo or a Supra for a racecar. The craziest I would ever go would be an 18g with meth. Keeping it conservative and on the stock motor. Either a PPG or MFactory gearset, get crazier with suspension and brakes. But not much more than that.