An .18 would be hard on the tranny. You can beef it up with metal gears, but it will still be hard on your truck. I've got GTs with 12s and 15s. I have a GT with a mugen MT 12 on it. I mostly bash at a private track and let me tell you, that MT 12 is waaay too fast. Of course you don't have to pull the throttle trigger all of the way back. I guess the main question that we all ask ourselves at some point is, how fast is fast enough? I'm guessing that you have the stock AE 15 on your truck. A high performance 12 will smoke that 15.
By the way, what part of GA are you from?