I wouldn't have posted it if I didn't want you to take the idea and run
For the wheels, heavy is good if it keeps them from losing traction. Now that I look at the design again, the upper (second) mousetrap should be tilted forward a bit so that the initial pull will transfer more power and accelerate it a bit more.
Just keep in mind that the first couple of feet are the most important. If you're going faster than the other guy after 3 feet, you're going to win, just like a 1/4 mile drag in which the first 60 feet are crucial. The big thing is that the continued power after the first trap will put your car way above the level of anyone with a 1-speed