uh... I think a looser diff provides more traction off power but a tighter rear diff provides more acceleration. Real cars with a lot of power, real wheel drive and a limited slip diff put the power to the ground but one must be careful with the power as the rear can swing out on you (over steer). RC gear diffs (gas cars on-road) usually use thinner diff oil in the rear for more traction. So a tight ball diff has less slip and puts the power to the ground vs. a little slip that gives more traction off power. Basically one has to find their own "sweet spot" for their driving needs.
http://www.rc-truckncar-tuning.com/s...setup-book.pdf <<<page 33 to 36
LOOSER REAR DIFFERENTIAL SETTING
A loose setting increases rear traction when cornering, but it may also cause the diff to “unload” more easily on high-traction surfaces that cause the car to unweight its inside tires in tight turns.
TIGHTER REAR DIFFERENTIAL SETTING
The car will have less rear traction when cornering because more force will be required to activate the diff. If traction isn't sufficient to activate the diff while cornering, the inside wheel will have less traction as the diff tries to turn it as quickly as the outside rear wheel.