Um... i may be crazy but....
I think we are making this much more complicated than it should be. Load calc in an old house can be somewhat tricky, but pretty much straight foreward. The contractor in questions seems to be trying to make up for stratification of air by adding velocity at the outlets to get a mixing effect. Where there must be a floor return which are very inefficient.
Also you can throw the fan curve in the garbage. Fan curves are developed in a controlled laboratory environment. They do not take into account what we call system effect, which is the effect the duct run and restrictions in that duct run have on that system. You will get more static from more airflow not less, so if the return is choked down, the result will be less amp draw on the motor. It will not turn harder to pull air through the return, it will pull harder when unlimited return is supplied to it. Cover the filter and amps will go down g'head try it.
Also forget power factor, that has to do with the horsepower.... on a direct drive motor it is going to do what it is going to do. If it were belt drive i would then worry more about the amp draw, service factor, power factor and horsepower. But this is direct drive so dont worry about that either.
sounds to me contractor was trying to overcome expensive problems in an old house. furnaces you put in most likely have higher blower speeds which are pushing more air, resulting in more amp draw, But at least an attempt at comfort. :)