Should check this out:
http://www.the370z.com/track-autocro...agstrip/93770-
going-square-how-did-you-like.html
As a practical matter, you're asking about the "lightest and fastest" size of a wheel. Well, the lightest size of the wheel is gonna be a shortest, thinnest one, its common sense; more to the point, unless you are tracking in a pure race scenario, you aren't gonna feel the difference in WEIGHT between a 9.5 or 10.5 inch wheel
The track folks run 18x10.5 +18 all around
They do it so they can run 285 width tires (or something close to that) at all 4 corners, in a square setup. The Z needs front tire to fight understeer, probably doesn't hurt to have braking grip too
To fit that size in front they run negative camber, which is also a positive for handling
Adjusting camber requires adjustable front upper control arms, such as SPL parts or Voodoo13. Probably should roll the fenders anyways, as its pretty easy and opens up plenty of space
and i suppose i should say it again:
18's are super small on the Z. They are what everyone runs at the track because it's lighter and opens up a wide variety of tire selection for better prices than 19/20s
for aesthetics, the Z really needs 19s or 20s
If you are just bombing around town going to the grocery store or school, i don't really see the point