Some more accurate math...
96 sq inches of duct relates the volume of air pushed through it (which is whatever the fan pushes and is not a function of duct size) to the speed of the air (given in the US in Feet Per Minute or FPM) and the loss of pressure as you get further from the fan.
Actually if you want to get technical about it isn't just square inches of the cross section. Rectangular duct behaves differently than round duct of the same cross sectional area because of turbulence in the corners, but only the geeks at ASHRAE (and I mean that in a good way) care about things like that.
Regardless of duct size or shape, CFM has no direct relation to BTUs. It depends on what temperature the air in the duct is. If you blow 65 degree air in to a 65 degree room, you're not doing any cooling or heating, you're just blowing air. Changing the temperature of a space is a function of both air temperature and volume. You can have the same effect with a lot of slightly cooler air as you would with a little bit of much cooler air.
Don't listen to this guy. All change-outs have to be tested by a third "parity" for a reason.
Sounds like you have a bone to chew, or point to prove, or something. Don't mean a thing to me.
Trying to just help the guy out by telling him the duct is to small for the stuff they sell today.
Customer has a small house 1000sf. main duct sizing is 8x12, according to my charts max cfm is 400. The lowest setting on the furnace is 757 cfm. I don't want any temp rise issues, (not enough maybe) or excess noise. Can I cheat, so to speak, by using a denser air filter and rechecking?
I am not going to re size his duct-work for him...
Try the following could be a simple solution to your problem:
Simple Systems™ CC750-115 Comfort Control ICM
the answers to the real questions are hidden from man.