ORIGINAL: BAMF90
Coverts to one speed, and removes the extra "weight".
Actually thats wrong on both cases. It is still a 2 speed as its placed after the 2 speed in the Revo. And it likely weighs more since the part its replacing in the FOC kit is just a plastic gear and main shaft.
A Center Diff isn't used to prevent wheelies, it just will if tuned right. A Center diff allows for bias braking and power control. So you can have more front or rear brake in your truck to be able to tune it for better driving. Also the power from the diff will go to the wheels with least resistance, therefore stopping wheelies usually, but thats really more dependant upon the diff fluid and traction levels as well.
Wheelie bars are fine for bashing on the streets and for people who think wheelies are fun. The Revo is now far more race than the Tmaxx ever was/is.