I don't want to drop the model of the car I tested (until after the mail ins!) but I took a stock 2011 hot wheel this weekend and tested some modifications...simple but effective.
The car is a metal bottom car. I had two of the same car. Ran them against each other about 5 or 6 times until I had a consistent read on which was faster. The faster was a full car length ahead of the other at the finish line of my track.
I took the slower car and made some simple mods to it to see if it would get faster. Took off the body and removed the spoiler (purely looks and a small hint of the vehicle). I fixed the axles with epoxy. I packed clay into the voids between the chassis and seat insert. Not a significant amount but got some in there. I then put the body back on and went back to racing.
This time the modified car was a good half length ahead of the stock car. I raced them again 5-6 times and got the same results. Quite a difference!
I'll post the car and photos later in the week...don't want to give anyone an unfair advantage on the mail in race!