I broke the stock sedan diff, chipped off some of the teeth in the ring & pinion . It was with a stock A15 and four speed.

It wasn't from abuse, I drove it fairly easy. I reckoned it was the extra torque of the fifteen along with the steep first gear of the four speed. I changed the diff and after a short time this second one broke. That's when I put in a 5-speed and A14. The stock diff lived under that combo quite well even under hard use.