...and to make this a little more 'real world':

If you go from 17 to 18 inch wheels, you're effectively moving the weight 1 inch outward from center. This means a 5.88% increase in diameter. Since the inertia increases as the square of the diameter, you're increasing the inertia ~ 34.57%. So you'd have to decrease the weight of the wheel ~30% or so to get the same inertia. If your 17w/tires is 39lbs, that means you'd need an 18w/tires that weighs roughly 27lbs...hard to find in an 18...

This is an approximation that doesn't take into account a few things, but it shows you how drastically moving a small amount of weight away from the center of rotation can affect the inertia of the wheel. It also shows that a weight decrease of ~2lbs probably won't make up the inertial loses of a larger wheel. But if the grip and sidewall stiffness make up for it on the track, this is all pointless...

 

get a nice set of 18 for daily and nice rubbers for your 17 for track.

I am picking up a set of 17 for my daily and keeping my stocker 16 for track.

This way, you'll have best of both world

 

One point that's been completely overlooked in this discussion, is that with 18" wheels, you can have 14" rotors (355mm).