Two very good counter points that should be considered. Those issues mean net cost analysis, Greenspeed probably will be more. Guess it depends how much heating cost savings there are in that big COP jump.

He'll probably not be putting in one much less two Greenspeeds, but without them in the cost/benefit analysis, the decision matrix is incomplete and biased. Poor or biased perspective leads to bad decisionmaking. If it were my audit I'd be modelling them to understand net cost.

But the home's lower level was cool even with supplies off and +60% RH,
TB - Infinity allows scheduling fan settings. Try managing fan settings seasonally.

My mother complained of icebox basement as well. We found that if she turns her fan to "On Medium" in April, then back to auto in July it seems to solve her icebox basement problem. I think she subsequently has gone to "On Low" the rest of the year, it keeps parts of the house from getting stale and dramatically cut dusting.

The idea was that by improving air circulation/mixing through the house during the period equipment otherwise seldom runs, and that the ground is coldest (pulling a lot of BTU from the basement that aren't being replaced), it evens temperatures throughout and you actually gain some cooling benefit to the rest of the house from the cold mass.

This assumes you have a low return in the basement, you want return pulling from the coldest location (hopefully Brian won't see this).