Bryant Plus 90 398AAV -- Not Enough Heat
I'm confused. A tempereature rise should be taken at the return plenum (or grill) and ONLY at the furnace supply plenum. The difference in those two temperatures is the rise. Outside of that when you measure the temperature of the air coming out of each vent, let's say the one's showing 85 degree air, then that is temperature loss and that would be duct issue.
31 degrees is not a good enough rise. Blower, if not on lowest speed, could be lowered to increase air temp and thus temperature rise but regarldess, it appears that you have ducts leaking or not insulated or something with the ducts happening.
You can expect to lose 1 to maybe 2 degrees at best, not 15.