I've ran 15's and 17's for track work, and it has to be said 15" 888's are alot cheaper than 17's if you want to run a spare set of wheels to keep for the track only.

I used to run my 17's on the road and had the standard 15's on 888's which i took to trackdays, but i fancied trying the 17's so now have them shod in 888's and tbh i've found them better on track than the 15's, more grip and more traction out of corners. As i run coilovers i also wanted to use the 17's as when the car is corner weighted iit will throw it all out using different size wheels.

If you're using yours alot on the road i'd definately buy some cheap track wheels and get some road tyres on your 17's, i don't think you see the full benefits of 888's on the road anyway as you never get enough heat into them!