I don't know if this is relevant, but in the German F3A-X class I believe the rules were changed for the 2009 season.
(F3A-X is the German equivalent to IMAC. I am not German, so this is just something I read, and is quoted from memory.)
Previous rules were max. 10 kg, the new rules have no weight limit.
In the new rules the models are limited to 2600mm wingspan. These are "scale" models, with max 10% deviation from scale, (same as IMAC) so fuselage length will be a function of wingspan, hence no requirement for further dimensional limitations.
In addition, there are limitations on the engine size/power.
IC engines: max 110ccm.
Electric motors: max 42V and max 8 kW static.

I don't know how they measure/enforce the power rule, but presumably they have sorted it out.
So it is clearly possible to have engine capacity limitations combined with electric power limitations, if one wants to.

Magne