[Paulbstockton6@hotmail.co]

17x20 is good for about 1600cfm and 12" is good for about 500cfm..... that checks out

Grille size 17x20 = 340sqin x 2 = 680cfm... problem
So the grille for the 12" round duct (capacitu = 500cfm) vs grille (680cfm) good.
Grille size for the 17x20 square duct (capacity = 1600cfm) vs grille capacity (680 cfm)..... not so good and this return is noises than the other.one.
So you have. .500+680cfm= 1180cfm. (Choked) for a 5 ton system which likely set to 1500 to 1700cfm....
What just happened...so now you will have excessive pressure (big deal, I'm in the same boat) This will make the blower motor work hard...drawing more amps.
I predict that if you tested static pressure you will be exceeding the .50wc spec.
The vents on the roof top are sufficient...the soffit are not. The total vent area, say 20x.5sqft =10sqft. You need roughly 10sqft soffit area, spread about the perimeter of the roof for your attic to vent properly...rather, the free area of the ridge needs to equal the free area of the soffit at the perimeter of the attic floor.

R30/38 would be minimum and I'll bet they recommend double that in current codes for new houses.

One problem with the ECM motors is that they do adjust speeds automatically....based on static pressure.....oops.... weren't we talking about that earlier.. I'm predicting high static pressure due to restricted return air....This might be an issue/might not.

So, since we are deficient in return air, look at the return plenum, or that 17x20 duct....can you tap into that and run an additional return somewhere where it might be benificial....?