Nothing beats "DD"
In fullsize cars its common to say that there is a 10% loss in FR cars.
20% for 4WD. (no sources but common estimate)
A lot of it is probably down to the bevel gears in the diff.
Usually its a bit worse on full size cars because they sometimes have the input shaft below senter to save space.
Hypoid gear.
All that said i think the main issue is lack of correct gearing for Mantisworxs
The evolution of brushless has been towards lower Kv and higher efficiency. good example is X11 - X12 to the new X20.