What I said might be completely wrong. An 18" wheel and tire combo might be better for track use on a Z32. However, without a race engineer coming into the discussion and saying "yes, it works better for these reasons", I won't just start saying that 18's are better.
When you say something is "better" than stock, you'd better have some damn good reasons to back it up. "All these other guys do it" is not a damn good reason.
Obviously you can come pretty close to the stock weights and intertias with aftermarket wheels, but coming close and being "better" are two completely different things.