I'd have to assume it would be more efficient to use a hash heater as an auxiliary unit where it's heat was directly supplied to the space.

Ideally, you could create a central system that would switch between the heat pump and hash heater optimizing for whichever was most efficient at the time, or to save money on asics, simply use a cheaper miner as a supplement.

At a certain point, the money spent on the hash heater is basically wasted if the time it is used drops below a given threshold. At the very least, your ROI could be severely effected of it really doesn't end up being utilized much.