The power saved would be minimal, have you considered adding a filter to add restriction while filtering the air?
I have thought about making a filter-casing (dust collector).
Its not about saving energy, but reduce possible fan-noise/motor-noise.
Now you made an idea into my head.
If I make a filterbox of wood, it will make space for 2 fans side by side.
Not made the decision yet, to have the fan blow heated air into the kitchen, or transfer (colder) kitchen-air into the heater-pump area, thus forcing the heated air into to kitchen door....as return-air.
I actually dont understand the sentence - "...adding a filter to add restricton, while filtering....".
I guessed you meant air-filter/dust filter...?