Rodney, as you know, I have the exact same issues...same fiver etc. I removed the panels in the basement to give me access to the furnace ductwork under the bathroom/bedroom. Here is where it gets interesting..(roll eyes!!) The ductwork is connected to the upper floor in the fiver by two separate lengths of round 4" ductwork similar to dryer vent hose (just cheaper material) One hose goes to the bathroom, the other to the bedroom register. The first thing I noticed was that each length of hose was 1' to 18" too long. In other words, the hoses kind of formed a loop on their way to the upper floor registers. Common sense told me this was not very efficient, so I shortened each length of hose so it formed a straight shot to the register and re-attached them using that silver foil a/c ductwork tape. That being said, I have not used my fiver since I did this but it appeared to be letting more air to those registers. Unfortunately, the way the ductwork goes from the lower main floor to the upper bedroom/bath floor is very inefficient. All the factory did was cut two 4" holes in the top of the lower ductwork....one right after the other, and then route the hoses to the upper registers. Does that make sense? When I was down there, I could not tell where the main floor ductwork ended or if there was anything in the main floor ductwork to deflect air into the hoses that go upstairs since the ductwork was actually below the floor and linoleum. I suppose you could take off the upper hose and stick your hand down into the ductwork to see if it actually ends or if there is some type of deflector but I didn't do that at the time. Now I wish I would have taken pictures but I didn't....
Overall, I think the main issue is just a poor design the way the ductwork is laid out. This probably wasn't much help, but at least it might give you some ideas on what to look for if you do tear into it.