Well, I've tried it both ways (locked & unlocked) on many
different courses in the dry, and *for me* it is definitely
faster with the diffs locked.
I agree 100% about the difficulty in turning on snow with
both diffs locked, that's why you definitely need the
ability to lock only the rear diff; you retain full steering
control in power-on situations.
Certainly might not be the fastest way around a racecourse,
tail-out, but it sure is fun and that't why I do it!