This is not a motor forum, BUT.......

Shaving some off the trailing edge will help with amp draw in a race, or in other words make run time. A hole will only help a little so will serrating the brush. All this makes for less brush drag on the com. Shaving a small portion off the trailing edge reduces overlap and makes the best way to make run time. I never had a problem with 3300s anyhow.

I dont' think the brush should be narrower than .140 in a stock motor. .140 is the RECOMENDED width for a MVP stock motor with both side shaved evenly. Only go back a little ways so the brushes stay straight in the holder. (hoods)

We can also run lighter spring tension in 1/12 than the sedans because the amp draw isn't as hard so lighter springs (green/green) can help if arcing is not a problem. I usually run red on + and green on - .

To answer your question, I would start with .010 and see if it helps, then maybe .015" A brush measures .185 to .190 so no more than .020", or .5mm.