I concur with Chris,
I tried the geared diff on saturday and my car was near undriveable. With a ball diff it was much better. I plan on trying different oil setups but so far I've tried it twice on slippy floor and took it out immediately both times.
As for the RTR, a ball diff would require adjusting from the factory and frequent rebuilds for the customer, which isn't really ideal. This is why there's a geared diff, it only needs to be built correctly, no skill/knowledge involved in the setting up process.
Fabs
|