Friction Facts tested a Gates.
The report is free., although it may not be reproduced without permission. But I think it's safe to say that the Gates dissipates around 3-4 watts depending on power. Download the report for more details.
On the use of the power meter for drivetrain efficiency tests: I agree that it would work if the conditions are similar, especially ambient temperature, but as pointed out not for an absolute efficiency measurement, but as a comparison between one system and another at the same cadence and same transmitted power.