A couple people on rcgroups' Power Systems forum have built their own motor testing system. It is a device which mounts the motor on the front of a metal tube and has a strain gauge on a thin section of the tube. This measures motor output torque. Then a hand-held tachometer tells angular speed, so output power is known independant of prop efficiency. A standard wattmeter tells input power. The device I saw pictures of was good to about 1600W with a 17" prop, so not quite large enough for a 2m pattern motor, but it could easily be scaled up.