ok...its early so i apologize if this dont make sence (coffees brewing !)
...is the pressure too high...or too low???
2000 cfm requires very large ductwork for optimum performance...
with that said...i woud calculate what the capacity of your existing ductwork is....and try to regulate fan speed accordingly.
If removing the air filter improved your goal...i would try to increase the size of the filter. (its probly 1 inch thick??- modify for a 2 or even 4 inch thick)
the extra thickness provides more surface area...and less restriction.(and a bit longer life as well)
it doesnt seem the blower is starved tho...(heres where i dont quite understand your post)
aside from pressure-....is it loud(at the vents?)...is the house comfortable and even temps in most rooms??
CA returns should be adequate for the HA side...not sure if larger cold air will help...but it can never hurt to have more.
...just remember- dont ever put a cold air return in the bathroom !!