I talked to the guys at gearzone, they said m-20 are rated at 375, m-21 at 400 and no officail rating for m-22 . You must remeber m-22s were designed for road racing not the drag strip. Yes they have stronger gears but the case is the weak point. They stretch ! Doug Nash made a cast iron case for them years ago but its rare stuff now. It all boils down the weight and traction the more you have of either the shorter the life span
In reality?? I have a 427 thats making about 500 HP on a good day with an extra 150 of N20. I have never had slicks on it but the same m-20 has been in it since 1984.