if you want to go hard, I'd say shoot for 700hp on a stock block w/ cams. it'll hold for a long time w/ the right tune, and that's about the deciding point for the r-154 trans. it may or may not fail. if you treat it with care, it might hold up. as for the rest... I think all you really have to worry about is the clutch (drivetrain durabilty in mind)
my 87 hardtop w/ 400hp was quick enough to give me a hell of an adrenaline rush... the targa tops I've had (currently have 1) feel slugish though... my buddy that's owned 30+ mkiii's says the targas are 800-1000 lbs heavier
so, just saying you might out-do yourself.... maybe not. I just try to be realistic with the goals I'm setting.