Sorry I don't buy it.

All a limited slip unit is, is a set of spring loaded clutches, that try to hold the side gears to the carrier.

I don't see how it could cost any torque at all(other than the added mass the driveline has to turn).

For that matter, the sure grip unit actually does nothing, if the traction is the same on both wheels.

Of course there is probably some wizzy scientific explanationa as to why it is true.