So you’re comparing adding a shift kit to milling the internals to add additional friction disks, along with the other things ATS does to beef up their trannys? I ran my ATS 42RLE hard behind a 300 hp / 400 torque supercharged I6 - the stock 42RLE will not hold up to that. I know because I smoked the stock tranny behind that engine - which is why I went with ATS.
My current ATS 545RFE is built to take 700hp. They know what they are doing. As I said in my post, it wasn’t cheap but worth spending the money in my opinion.