Not really, that's a 75% efficiency.
You want to use LiPO? I hope you're going to pad it real well.
How much current that battery can put out depends on it's C factor. And the power isn't quite as simple as 400*50 = 20kW. The motor controller will eat up some of it.
I would not say that the controller is the problem, the losses should be far lower than the motor. Motenergy have announced a new motor with 30kw peak and 92% efficiency with 90 Nm stall torque. Thats in fact closer to a 701 then I imagined. Not in horsepower, but in torque. Its close to peak torque on a limited 701 with b-pipe.
The motor weights in at 35 pounds, not that much imo.