You've got to be more specific about your needs/application besides "track work"
- In general, for a car this heavy in Lemons, most people are buying a new set of tires each race... two max (we typically do one race, one track day, and sell the tires off @ 15-20% their value). For me, one hesitation to go with 17s vs 16s was tire cost (which might be mitigated now if we switch to Nitto NT05s vs Direzza Star Specs)
- Depending on your application, you may be limited in what tires you can run. Lemons has a 190TW rating minimum. If you were only using this car for short races and occasional street use, you might find going with a sticker compound to be better. I don't know about non-Lemons legal tires but I do recall seeing some Spec Miatas at a track day with tires so sticky they were picking up gravel just driving around the paddock. Even if you could "sneak them" into a lemosn race they would be at their cords before the end of the first day.
- Track racing aside, virtually everyone I know with 18" tires has replaced at least one because of a sidewall bulge caused when they hit a pothole with their rubber band tires. That's an expensive f-up. I've done it twice with my S4 w/ 225/45R17 and those Michelin Pilot Sports used to cost me $250 (when bought locally)
- Asthetics might also factor into your wheel/tire choice. I don't care (because $500 car) but you might. For years (until we upgraded to a V8, I ran with 15" spec miata tires which were 3" smaller in diameter. Even with a 2" drop, the tbird looked like a clown car.
- My "gut feeling" is that 17s are probably the sweet spot on cost vs. performance as Madmikey pointed out that the Nitto NT05s he ran were actually cheaper than the 16" Direzza Star Spec ZIIs and BFG Rivals I have used before but without specifics on what you plan to do, I'm not sure if I have any better guidance to share with you.