I use a Nibe 11kW air source heat pump (ASHP) to heat a swimming pool. The only thing I would suggest is that you check the performance figures for the Mitsubishi, which should show, for different air temperatures, the Coefficient of Performance (COP) of the heat pump at different supply temperatures. The COP decreases as air temp. drops. I'd just be sure that your client realises this, because most of us want most heat when the air temp. is low, which will be at the lower end of COP. We only use the swimming pool in the summer, so COP is about 3.5. If we'd wanted year round performance, we would have gone ground source. (Apologies if teaching grandmother to suck eggs.)