Yes it could but I don’t think that’s it. The system needs to be able to run at 100% output at 60 degrees outside temp. Do the math at the projected flow rate and known delta T and calculate btu output.
I don’t know if I trust the heating performance chart. Why would the system efficiency increase as the return temp increase???
Really you need to be able to measure face velocity and static pressure.
I did some quick math. This morning it was about 20 deg F outside.
I'm keeping an indoor temp of 67. I saw temperatures when the unit was running full-out of about 104 deg. That's a delta of 37 degrees.
At 20 deg F outdoors, and 70 deg F return temperature (close enough to my 67), it should be delivering about 51000 btu/hr.
Using q = CFM x 1.08 x DT, this calculates a flow rate of 1276 cfm. I can't be entirely sure, but I think only one zone was open at that time, so this is consistent with the 1300 cfm I measured with static pressure yesterday.