A lot of people seem obsessed with speed (I prefer torque for better acceleration - better than high speed in racing on tight courses). Here is a mod that will give you lotsa speed - maybe even too much:
Use a 145mah Lithium Polymer rechargeable cell. Since these are 3.7 volt nominal they will really add some punch to your car. They weigh the same as a single nicad or nimh cell. One downside is that they need special chargers and you have to be able to disconnect the cell so it does not discharge completely. They die if they go below 3 volts.
Cells are available here and you can learn more about them as well:
http://www.fmadirect.com
Have fun
Michael