I have both the 12" Cobra kit and the 13" Cobra kit. The jump from the 96 upgrade to the 12" Cobra was a HUGE improvement. The difference between the 12" and the 13" was there, but seems less significant than I would have expected. I do think the 12" kit gives you quicker acceleration since it weighs less, and also allows you to use the lighter 16" wheels.
We have sold a number of these 12" kits to people, and they always get great reviews.
- Mike