Can fine tune them, but your average user of them has no idea how they work, just fits whatevers given to them, and leaves it, then after a few years the plates are worn out and its as bad as an open diff (or stock shite viscous LSD) again.

And yeah, decent plated diff will be quicker on the track, but he never mentioned a track once.
And compared to the wank stock viscous LSD, once you adapted your driving style to suit, its prob faster...