i'll chime in regarding the vsld and my experience. i've been running the q diff in my z for nearly 5 years now and about 20,000 miles. multiple track days each year including, mid ohio, daytona and watkins glen and have never had it 'open' up during my sessions. i like the fact that it is extremely quiet, especially when i compare it to the nismo clutch lsd i used to have in the r200 which the q diff replaced.
i have heard on tighter tracks the vsld will overhead and become an 'open' diff until it cools sufficiently...