What tire sizes should run on my gd3 fit to maximize performance? My wheel specs are 15x6 +48mm

 

I’d run a wider wheel than that personally.

195/50/15 is too wide for 6” wheels in my opinion but is probably most okay.

205/50 would be ballooning on a 6” wheel.

The tire choice is more important. An all season in the biggest size possible is going to get outperformed by a narrow grippy summer tire.