i dont want to comment on guesstimates of transmission losses,as it will vary from car to car,so you can only give a very loosely estimated figure. as to 2wd v 4wd, the skyline gearbox in my escos can vary the torque split from 50 50 to 100% rwd. on the road when moving the rwd mode feels a lot faster on acceleration even than a 80 20 torque split.