Check if you haven't locked the slipper clutch completely. Normally the slipper should be the artifically weakest point in the drivetrain. It's perfectly normal that gears break more easely when using a one-way diff. That's why I can't use one on my NTC3. The drivetrain won't survive a 30 min final and AE refuses to design metal gears.
I also noticed on this car that it's easy to miss the fact that the gearbox screws aren't completely turned down. Always check this visually.