This is one reason why Lehner motors are so highly regarded~good part-throttle efficiency. I guess its due to the stacked magnets on the rotor, which i hear Lehner bought the tooling to make in-house. Im pretty sure all of lehners motors use stacked magnets, except maybe the basic series. Large hackers use stacked magnets, but the smaller ones dont.
I wonder if stacking the mags in an outrunner would do anything at all. I also think bell material plays a part, I get better results using pole-piece material out of high end loudspeakers than i did using iron pipe, my poor little lathe can barely keep up making a 1mm ring out of a solid chunk of steel.
Ive also wondered how the lack of an angular contact or thrust bearing on the back of most(if not all) outrunners affects the power to the prop under load. I know on a cnc router chaning out single row bearings to angular contact on the leadscrews made a huge difference in the smoothness and operation of it. They make em in 3mm id, pricey though. http://www.vxb.com/page/bearings/PROD/Thrust/kit1027