Ok, I find the G to be a bit dated looking and not quite as sexy as the Z. Also, there's something to be said for not having to lug a bunch of people around in your sports car!
I average about 22-27mpg in between city and highway (about 50% each).
Mods: Stillen CAI ~$560, a good cat-back exhaust Fast Intentions - ~$1200-1300 and some would argue for high-flow cats as well ~$300-400. Lowering springs - Swift is a popular option ~$300, and some 20-25mm spacers to bring those stockers out to the edge of the wheel well ~$300.
Then you could go hog wild and get yourself a fat set of Venaci Spirit wheels - ~$4000.
So for an extra 8 grand you'll have yourself a sick ride!