Are you using the standard 20T pinion? is everyone else?

The batteries could affect the speed. The last batch of nimh that I used were pretty bad compared to a lipo, especially if charged the night before.

Have you got access to a dyno? get someone to dyno it for you. The black can motors were all over the place, some were pulling 19,000 out of the box, and others up to 23,000, that will make a huge difference down the straight.