What is the max torque and horsepower a M21 can take before it starts to breaking?

A lot of that question really depends on how you intend to drive the car. Street, Strip, etc. In my opinion an M21 can handle street driving with just about anything you can throw at it, but it is definetly not a tranny you are going to want to get on day in and day out and the drag strip.

I had close to 400 HP in my last car and the M21 was fine. I have 600+ HP in my current project and due to the way I drive, I don't expect any problems.