I like those 15's. My SI's are treating me good. I've gone from 17's to 15's to 14's to 15's again. I'm a big fan of 15's or 16's myself.

 

Agreed, 15s and 16s are best on the street, mainly because they look good. Performance will similar between both of them.

For an all out race car though, you'll want to go smaller. As low as 13" with the widest tire and lowest profile you can fit.

 

my car is lowered about 3 inches on skunk2 full coilovers