Motor... driveline... you're probably right. And maybe somewhere along the driveline, anti-squat will come in and play.
But I think this entire 3-versus-4-gear business isn't about drive train torque, that's the same in any case. I think it's about the rotational inertia of the armature ... ... and pinion. Because they spin so much faster, their combined inertia is much bigger than the inertia of spur, slipper, diff,... the only other factor big enough to be significant being tyres. (tires to you, Paul
) Anyway, yeah, inertia. The thing that makes a motor jump out of your hand when you hit full throttle holding it, that 'kick'. I think it does matter *where* the kick happens. The closer to the rear axle, the bigger the (instantaneous) weight transfer.
I think this is the reason the XX4 flies the way it does, with the motor spinning 'the wrong way' (IMHO) but being way forward.
***Disclaimer*** Been a long day, brain in neutral, fingers in gear.