somehow it seems difficult to understand that the 2-speed is intended with the stock engine and if you want to use anything more powerful (a race .21) then the single speed (regular center diff) is the choice.
I feel that the IGT class is not only about going fastest but having fun together and hear/feel car shifting is part of this fun.
If you really want to go fast then Evolva is the option as it has the gear box you need to do so.