Funny you ask, I'm running Porsche wheels on my Z too
You'll need two pairs of the H&R adapters and they'll bolt right on. You'll want to change the tire sizes though since Porsche tires are too short.
I'd run 225/45/19 or 245/40/19 fronts
285/35/19 or 305/30/19 rears
The front fittment will be a little weak, kind of how 15mm spacers look on the stock front wheels.
Rear will fit very nice with stock alignment, stock ride height or lowered on springs. Should come out a bit more if lowered on coilovers with some extra negative camber.
Your final offsets with the H&R 25mm spacer adapters will be:
19x8.5 et 31 front
19x11.0 et 26 rear
The only bummer is that the stagger is so extreme front to back... It would be better if the fronts where a bit wider since the Z really likes a bit more tire up front. It works well on the Porsche due to the rear engine/ rear weight bias.