oddly enough I JUST fixed a system with a similar problem in split level home.
lets see if I can describe the problem, what I did to fix the problem will be in RED
Original Equipment: 2½ton Whirlpool approximately 25-30YOA, air handler in the lower attic, with a remote box plenum in the upper attic. the upper attic handled the 3 bedrooms and a full bath. lower plenum went to the living room, dining room and kitchen. (all supply runs are R-4 insulated flex)
Supply to dining room=8"--increase to 10"
supply to dinning room and kitchen=8" split to two 6"--added two separate 8" runs
supply to upper remote box plenum=10"--added a 12" supply to the remote box plenum---->Off the remote box plenum
supply to Bath and bed rm#1=8" split to 6"--remove the 8" and added two separate 6" runs
Bed rm#2 remained one 6"
Bed rm #3 = 8" split to two 6" -- removed 8" and added two separate 6" runs
(all added flex are R-8 insulated)
Return flex is 16", centrally located at the top of the stairs---left it as is
Customer complaint was there was almost no air coming from the registers, and the a/c was toast
I installed a Rheem Variable speed 2½ton condenser with a 2½-3 ton air handler. increased the supply runs as stated.
customer unbelievably happy, said the house has never cooled this well in the 3 years they have owned the home. They said "we have never felt so much air coming from the registers"
The point is, if you have one room with little to no air flow, the only fix is to increase the duct and increase air flow to that room.
Increasing the size of the equipment won't help you out unless the system was undersized to start with.