That didnt help Dave Willard in the SMA Mini Enduro. I ran a ball diff and it was awesome. I won by four laps.
Z-Mann is right though, ball diffs are expensive. With a normal gear diff, you have to loctite the three small screws that hold the diff together and lube the gears with anti wear grease. This stuff is sticky as and will slow the differential action and ultimately aid in getting the power down early when exiting corners. A ball diff will help on corner entry,and you will have a less chance at traction rolling but if you power out to hard you will wheel spin and therefore slow exit speed.
Personally i like a ball diff, but it comes down to the way you drive. Mini's are about throttle control.