An update:
Some Quaifes can have problems from the factory. I know of two, one of which was the second one I purchased. The diff tends to go open much more easily then it should. In my case, with 300hp pushing it, it would go open all the time and the car basically undriveable around a race track. I sent the diff back to Taylor Race Engineering and they sent it to Quaife UK. Quaife said, "Nothing wrong." Taylor had already fixed a Quaife diff in a Viper that showed the same problem - they increased the internal preload on the diff. I have yet to test the diff they sent back to me with that fix.
Another point:
Suspension changes with the Quaife tend to be more drastic then I originally thought. I want to do some more testing, but I think I'll be moving to a more east coast Z suspension setup (no rear anti-roll bar and bigger front springs) to see if that helps put the power down better.