HD For Indies

the elevated luma is probably just a gamma shift (since fcp corrects video gamma in qt files and most comp apps dont) and the noisier blacks is most likely the rather heavy despill that is built into keylight, resulting in more noticable artifacts.
doing the despill outside sometimes helps a bit, and primatte usually works better for me anyway, but in the end you'll just have to avoid spill while shooting or work hard in post.

++ chris