Vito,
What temps were you hitting with the 12?
I run a 13, but thats on a track. There will probably be less load on the street and more air flowing over the motor so you may get away with a 14. But I think 14 will be as far as you should go with a 4.5 turn motor for pure street driving.

When you said that it was running cool, was that after 10+ minutes of driving? Maybe even driving it until you get to the esc voltage cut off?

I know you said that you don't drive it for the entire battery pack, but you want to gear with the assumption that you will. Maybe after driving with a 15t pinion for 5 minutes you are getting to 160°, thats good... but what if one day you are really having fun and drive for 10+ minutes with the 15 and get your temps to 210 and fry your motor?

Gear it to the max acceptable temp at the maximum run time your battery wll give you.