at 0 rpm, you can have all the torque in the world, but, no power. Power is a function of torque and speed. At a given torque, hp increases with rpm.

So, even though torque drops with speed, I would think that it drops rather slowly. I doubt that most motors running at 200 rpm make half the torque of one running at 100 rpm.

Therefore, it does stand to reason that a motor going through a chain drive taking advantage of the ability to run higher rpms would outperform a hub motor. As for efficiency, there is some loss through a chain, but, I doubt it is anywhere close to 15 pecent, unless maybe you haven't lubed your chain in 10 years.