I don't know Porsches at all but every die hard track guy I know says they're highly optimized just stock. Luke@tirerack told me you'll spend a ton of money on suspension mods to the Z to attempt to bring it on par with the stock Porsche ON STOCK TIRES... and he's owned and upgraded both.
I find the RUF CTR "Yellow Bird" rather inspiring to watch from back in that era. That may be an unfair comparison and of course a ton has to do with the driver himself. Again, I really don't know porsches or suspension mods yet. I do know that the Z (and Corvette) has a tendency to spinout both under power and upon braking.