The reason the screw works on a TC and fails on a Mini has nothing to do with the power the motor is making. In the Mini, the idea is to tighten the diff way tighter than you would in a TC to get it to act more like a limited slip diff. People also use AW grease in the diff to get the same effect. In a TC, you want the diff to be just tight enough that it doesn't slip so you tighten it till it's snug then back it off till you have the right tension on the spring.

I see no reason to cut a slot in the screw. Every one I have seen is tightened with a hex driver.

Andy