Maybe you can help me. My home has 2 zone heating/cooling. The AC units are 3 ton and 3.5 ton. I live in Arizona. Liquid lines were swapped out last year with 3/8" since 1/4" was the wrong size. The return vent hoses (1for each zone) are 16" round. The 3.5 has a 20x24 diffuser vent cover. The 3 ton has a 24" square one. I had readings on the 3 ton unit. He measured 575cfm. Would it help my efficiency to run an ADDITIONAL 12" flex return line to the air handler?

Yes it deffentally would you need to have 400 cfm per ton of cooling, so 1200 for 3 ton and 1400 for the 3 1/2.

16 in duct is supposed to carry around 1400 cfms if it is installed right with no kinks or bends. How ever inless you have a free flowing return air grill I dought the registar is allowing that much. Is you filter in the air handler or at the return grill?

The filter is on the grill section. Would I benefit from installing a secondary 12" return vent? Thanks!

Yes you will get lots better performance from your system. I would think that a 10 in would do you fine.

I might add to the above posts that in order to move air one must also take into consideration the size of the supply ducts as well as the return, a sixteen inch duct only moving 575 cfm tells you that you have other issues. Good luck, Mike