Can't give you a specific answer, but I can attest that it is a real problem.
A few years ago I built a custom home for an older, extremely difficult client. He insisted that I bid the project without plumbing or HVAC, since his good ole buddy was the greatest plumber/HVAC guy on the planet. Didn't take long to figure out that this outfit (that we nicknamed "dummy and daddy") didn't have the smarts to build an outhouse, or put a window AC in.
Long story, but the bottom line is that they grossly undersized the returns and the furnace self destructed before the warranty expired. the factory service guy shows up, says it's a wonder it ever worked in the first place, and that they would only honor the warranty IF my customer hired a professional to re-do the ductwork. $4500 later, my client got a fully functional heating system.
A competent service company should have the equipment and knowledge to determine exactly how much of a return air deficit you are dealing with, and if it can be addressed without too much rework. Good luck.
