The factory reciever cuts the voltage to the servos from 6 to 4 volts. If you unplug the reciever battery pack from the BATT port, and plug it into the AUX port, you get the full 6 volts to the servos and they work much better.
I removed the shield on the spur gear because it gets in the way. I also moved the primer bulb to the left hand side of the chassis so I can get to it without reaching around to the other side of the truck when starting it with the body on.
I upgraded my transmission with an aluminum 2 speed pinion gear set out of a Savage. You have to do it right, or you end up with problems.... but my ported 21 has drank a gallon of 20% and hasn't had any problems with the aluminum gear set.... Even though my buddies truck broke the stock plastic set within the first 1/2 gallon.
Removing the spring on the brake linkage, and replaceing it with rubber fuel tubing drastically helps the truck stop faster. I did away with the spring all together, and set mine up solid. If you adjust the leverage point of the brake arm on the tranny, you can adjust your brakes to make the truck lock all 4 wheels if you wan ti to, with all stock brake hardware.
I also installed an Ofna fail safe on my GT before I ever drove it, Ofna makes one that is super small and works great. It already saved my buddies truck when someone didn't clear thier channel before turning on thier radio which was on the same channel. If you plan to run your truck with the stock AM radio, a fail safe is an excellent investment.
Other than that stuff, the truckx are rock solid. I love mine.