15's are GREAT! I wish I could run them in GS! As long as you can find tires to fit the 15's stick with them. If they are 15x6's you won't need to change wheels ever. You can eventually put 225 Hoosiers on them. Smaller wheels are always better for acceleration. Not only do they reduce your gearing they have less rotational weight so you have more power.

Here's some 225 Hoosiers on my friends Focus with 15x6's:
Focal Jet

2004 Focus PZEV 2.3L - KW sport suspension, SP camber plates, Eibach front swaybar, FS intake, FS Stealth exhaust, 3.82 Final, Torsen ATB, Prothane MM insert, E1 CSVT wheels 1/4 mile - 15.201@89.4mph