As a Canada only product, I suspect that the details of the Zuba are not widely known by the pros here. You should ask either Mitsubishi Canada or the local area distributer these questions. I dealt with the Ottawa distributor and he was very helpful.
I don't even have our Zuba installed yet and am no expert (10 days to installation - can't wait!) But as I understand it, the Zuba is designed to run at constant 1200cfm in heating mode and 800cfm in cooling mode. I also read that max static pressure is 0.8". So perhaps at 1200cfm you are exceeding or close to the sp? I think they told me that the air handler will automatically cut back if sp gets too high. But again, ask the experts.
I do suspect that by limiting air flow, you will be downgrading the capacity. I would have the duct capacity carefully checked. How long are the ducts? Maybe if you could post a diagram one of the pros could advise if the CFM should be limited?
One other thought - Have you had a Manual J or similar heat loss calculation done on your home? In our case we found we needed 42k btu/hr at -25C design temperature for that wing of the house. (I bought HVAC-Calc to double check it) The balance point turned out to be at 1degF, so Zuba has excess capacity down to that temperature. If the capacity had been limited to 32k btu/hr due to lower CFM, the balance point would have been at 10F. (Balance point is point where plot of heat pump capacity vs outdoor temperature crosses plot of house heat loss vs outdoor temperature. Use say 70F for zero heat loss)
I asked and was told my Mitsubishi that COP does improve as compressor cuts back, but no numbers were give.
Where are you located? May be interesting to compare notes if you go ahead.