Racers use .12 not only because of weight and they have to but you get quite a bit more run time on a tank of fuel too. The higher end .12s make just as much power as the .15 and actually can make more rpm easily because of lighter internal engine parts. Tire choice is THE single most important part of your setup but after you get that down work on driving (I don't know how good of a driver you are). These trucks take a lot of throttle and steering control and before you really worry about setup worry about feeling the car around the track and driving consistantly. If you can't drive than you wont even be able to tell what any change to the car has made anyway.