I don't know for sure, but I think in a straight line, with both wheels haveing traction, there would be no power/torque loss. While turning, the clutches would slip, to allow one wheel to turn faster. This would generate heat, resulting in a power/torque loss.

Come to think about it, if the clutches were doing their job as designed (to transfer power to the wheel with the most traction), there would be heat generated, and this would equate to a power loss.