efficiency goes bad when on part loads. it depends a lot on the combo and how its used, but basically its a big compressor trying to run a small head, so it short cycles. eg a 8kw lounge and 3kw bedroom means a 11kw compressor. if your only running the bed room its a 11kw compressor on a 3kw head. big miss match especially if you only need 1 kw output. variable helps a bit, and theres a few tricks that help.
So what are the long term implications of this scenario? IE our bedroom one runs all night, I forget the size, maybe 3kW and the compressor isn't as large as 8kW. Premature wear and tear on the outdoor unit, or just reduced efficiency?
It's been ridiculous how little our winter bill changed since they went in.
For the OP, I put in Mitsubishi, I didn't want any other brand, and contacted them to find an installer. A company called Apple Air did the install and I was happy with it.
NB wifi is absolutely brilliant too. The remotes went in a draw a week after the installation and haven't been used since. It's nice getting your phone out when you're about to leave for home and turn the heatpumps on if it's a crappy winter day.