Good thoughts. I already tried to check for leaky ductwork. Basically I cranked up the furnace and went around the basement ductwork with a feather to check for leaks. There are a couple small ones that I will seal up later with foil duct tape....but nothing that should drastically change the heat output.
As for the evap coil, I'm not sure that this furnace has one. I did some online research and determined that this is a Trane XB80 model. It doesn't appear to have an evap coil but an aluminized heat exchanger. I suppose it could be dirty but I don't know how to get at it to clean.
Fan speed seems like a likely culprit. The XB80 manual tells me this furnace has 4 fan speeds (but it doesn't have a wiring diagram or say how to adjust). I know it has something to do with the fan wiring on the control board. Right now the jumpers are set - *, *, *, *. I've heard black is high, blue is med, yellow is med/low and red is low. So is my current fan speed at medium? I don't really understand what COOL-H and HEAT-H are. This furnace doesn't provide a/c, only heat.