Have you had much track experience? If not I would stick to street tires for your first few track days and look to get a good instructor. You will learn MUCH more about the car and how to drive it around the track safely, in control and ultimately very quickly. If you have lot of track time then toyo R888, or some take offs from someone running Hoosiers in the size you need maybe a good choice (the latter being a very inexpensive option to run R compounds). Have not had a chance to look at sizes just got my IS-F on Friday. Drove it to VIR (GTlive Koni Challenge this weekend) today and certainly got a lot of attention in the parking lot. Cant wait to get it out on the track.
I am looking for an answer to this question too, so I'm tagging on to this thread. From what research I have done so far, you can fit 18" wheels on the IS-F and still clear the brakes, but I have not independently verified this with my own set of track wheels / tires. 18" slicks will probably still be a lot of $$, but less than 19" (if you can even find them...i can't).