Ever driven a live axle go-kart? Those things are dogs at low speed with the wheel cranked. Everyone guickly learns to accelerate in the straightest line possible. Reason is simple, in a turn, the engine has to not only accelerate the mass, but over come the friction of the two rear tires having to turn different speeds.
Now in a limited slip, the axle can differentiate, which GREATLY helps, but there is friction present in the cones/clutches, generating heat, all made possible from consuming torque from the motor. So yes there is some truth to it....IF (big IF) the differnetial is being forced to differentiate (its in a turn).
Straight line? I wouldn't think there would be a difference, and of course if the peg-leg set begins to spin......

...and as far as a 15% torque loss? I don't believe it.