Hi timmig,
I also race 1/5 with Greater LA. If you're planning on running indoor carpet 1/12 at OCRC then you may want to go with a 13.5 motor because that's the consistent 1/12 class we race on both Thursdays and Saturdays. Also you'll find that the GTB will be slower than your competiton because you'll need an esc that has some form of timing enhancement. There are a few guys who'll race mod with you, but they show up irregularly. For the majority, the 13.5s are very fast with some really close racing.