Out Runner.  They work!

 

In-runners run at much higher RPM than out-runners, and don't have near the torque. In-runners are frequently mated up with a gearbox to increase their torque, exceptions being applications like EDF jets that need very high rpm. In-runners are smaller dia. and longer length than out-runners and have higher kv ratings. For stunt applications out-runners turn our kind of props at our kind of r's.  

Simplified. . . .
Inrunners are constructed with the magnets attached directly to the shaft, which is surrounded by the copper windings attached to the case which remains stationary. Because the magnets are close to the shaft it spins very quickly. This means they produce high rpm but low torque. Inrunners are efficient and powerful, but need a gearbox to spin large propellers. Inrunners are best suited for use with smaller propellers for faster flying speeds. (Or Ducted Fan models for example).

Outrunners are constructed with the copper windings on the inside. The shaft is attached to the casing that contains the magnets which spins around the copper windings. Because the extra weight of the bell and magnets are further out from the shaft it acts like a flywheel. In general, outrunners have a lower rpm but a higher torque than inrunners. This higher torque enables an outrunner to spin a larger prop.

TTFN
John.

Hello Charles,
When all is said and done, the geared inrunner will be set up for the same KV (at the pro) as the outrunners we use now.
It would be more efficient, weigh about the same, but ...
Nobody makes a gearbox that will survive CL Stunt cornering loads.
 Regards,
Dean P.

Hi Phil,
All these motors have an efficiency peak, and it is often better than 90% ... but that happens at very light loads and low current.
It may just be a matter of specs-manship. Still, if you remember where you saw it, flag us.
take care,
  Dean