I am totally addicted to the 15" wheel kool-aid, the acceleration difference is repeatable and measurable. I just sold the last of my 16s, like you said tires were getting tough to find, I know 225-45-16 on a 16x8 would rub for me and 225-50-16 had no chance of fitting so I stuck with 205-50 and 205-45 on 16x7 but the extra 20mm I got going to 15s meant overheating the tires was much harder, I am lowered though so fitment will vary. My recommendation for the on a budget method would be 15x8 and 225/45/15 nt-01s which is what I run I agree 9/8 will be faster but I rotate my tires front/rear and left/right once a day because the wear is uneven and it lets me get the most out of the tires. If I was trying harder I would run A7s on 9/8 and do 3-lap stints but I wouldn't have as much fun despite dropping seconds off my times and importantly for my budget the $$$ adds up. You can keep the 16x7s and have them as your back-up rain/drive to the track set. That is what I did with mine (I ran 205-50-16 Z1*) And 'taller' gearing means the drive to the track is at slightly lower rpm.