Well it seems to be pretty durable. Ive seen a ls7 fiero with some pretty high torque ratings running the f40 trans with stock internals fine. And the f40 is only rated at just under 300 ft lbs to handle. But it seems it holds up to much more then that according to some fieros. But I did find a company who did a group buy with the hhr ss'* to build the f40 to hold more power. They have a stage 1 and stage 2 build option. Stage 2 is a full "revamp" inside that makes it hold up to 590 ft lbs.
They also offer the Quaife LSD as an added option as well.