Grooves in the diff rings aren't always all that bad but I always sand them off when I rebuild the diff. Sanding is the only way to get the rings flat (short of a machine shop).
I never run the inside and outside row of diff balls in the Kimbrough gears. It always seems to make my diff act all funny. I don't know if it's the extra drag of so many balls or that the inner balls turn slower than the outside balls. I just don't like both rows.
On a side note, I'm finding that the rollouts that Novak has on the site for 12th are a little "conservative". Ok, a lot conservative. So I've been working on a little spreadsheet to give a better starting point for 12th (and a GTB) based on what I was running at CEFX and a couple times at Grand Rapids. The chart is based on KV ratings so it might work with some of the other motors but I haven't tried many of them yet. I need to run a little more mod this year to update the mod side (I am the master of thermalling the speedo).
The sheet's just a starting point and you should check the temp but gear for LAPTIMES. Temp is just for safety. (the 21.5 rollout was just a curve fit I didn't run it)