Something must have gone really wrong when I measured power first time.
Just measured again, using a lamp as a load.
at 600RPM, it sees 18VAC at 85mA.
18 * .085 = 1.53W
When the stator losses are .45W, this means that .45/1.53 * 100% = 29% are stator losses. This is much more than the original 10%...
What I suspected was correct: my original powermeasurement was not good enough.
30% is a lot; hopefully it will be less for the full size conversions (130W & 3hp).