we have a 2400 sq ft house in missouri, when we first bought it new it had a builder grade furnace and ac. The furnace always ran and our bill was rediculously high. So we replaced with a bigger furnace and a/c furnace is a 30 kw and I believe the a/c is a 4 ton unit. We have never had alot of pressure at any of our vents before or after our new system was put in. Our main trunk line is 48' long and its 8"x20" the whole length. the plenum is 16.5"x22
the return air is (the box the furnace sits on) 28"x36"x10" then connects to a 10"x25" that goes up to the return trunk line which is 8"x20"

I wanted to know if I stepped down to 8"x16" or 18" for a section or two on the main trunk line for my supply, would that up my cfm to all my vents? I have asked several hvac guys nd noone will give me an answer or is willing to come out.
thank you for your help!! sorry if Im not using the right terms. thanks again, shawn