If your planning on doing track runs along with keeping your car a daily driver 17x7 inch rims with an offset of between 50 and 53mm are probably going to be the best in terms of performance and ride quality. With the 17's you're going to lower unsprung weight compared to 18 inch which also require a lower profile tire and that means poorer ride quality. If you have an uprated suspension and don't mind the loss of ride comfort, then 18 incher are going to provide better performance on the track because the increase in unsprung weight is going to be counteracted by the better suspension setup. In this case 18x7 inch rims with a 35 profile and an offset between 50 and 55mm would be the best option.
there are 2 theories on this, both of which are correct:
1) a bigger wheel is better, since you will have less sidewall and more rim, which leads to less flex. also, bigger wheels clear bigger brakes.
2) a smaller wheel is better since you're turning something that's a smaller diameter, and usually has much less rotating mass...which equals better acceleration. less mass also means the suspension and brakes have to do less work to acheive the same result.
in effect, you're also somewhat changing your gearing when changing your rim size. a smaller rim will have to roll faster than a larger one at the same speed.
i'm running my99 RS 16's on my car, and i find them to be too big in some cases, but with the 4 pots in place, i'm kinda stuck with 'em. like anything else, it's going to be a trade off. supposedly raceline RL7's clear 4 pots, and are 15". anyone know who carries them now that SPD is gone?
maybe it would help if you defined "track use" a little more clearly. for autocross or tight road courses, the smallest lightest wheel you can get away with is going to be your best bet. if you need high speed superspeedway stability, then a bigger rim would be a better choice. also, good tires for 17's and 18's are EXPENSIVE. if you're going to be chewing up tires on a track, that's always another thing to think about.
there's no supposedly about it. they definetly clear the STi 4 pots. they are rally wheels though, and weight is not as important as durability.
personally, i would go with either 16"s or 17"s, save some money on the rims and invest it in tires (but i'm a tire snob). the other issue about gearing changes in rolling diameter not mentioned is that with a larger rim, while you won't accelerate as quickly, you will have a faster top speed and will have to shift less. i suppose it really comes down to the track and your driving style.