Don't know much about the Dirt Thrasher, but I'll try and help a bit
1. Bigger pinion = more top speed - but other ways to get the most out of your motor is to make sure your wiring is good, and your motor is kept clean - I hope you have an electronic speed controller - the more expensive electronic speed controllers have lower resistance, which helps speed - also, matched, quality batteries perform better than cheap stick packs. If you're using Tamiya-style white block connectors, change them for something with lower resistance and better power handling like a Deans connector. And if you're using bullet connectors to hook up the motor, change to a directly soldered link. And solder the motor brushes on to the endbell, instead of using the screw-on tabs.
2. Lower c of g gives better handling (in general), but makes no difference to speed. Lowering the cars weight will improve speed though.
3. I assume that was lubricating silicone spray - it should be ok for the surface of the gears, but i wouldn't use it on the bearings. Upgrade the kit bushings to ballraces if you haven't already.
4. Basically, a well-maintained car is a fast car. I get much more fun out of racing a car around corners as well (there's nothing as satisfying as using a slow car that's well driven to prove a point to a fast car that's badly driven) - perhaps that could be next thing for you and your friends?