I highlighted in red what I would consider the most important concerning efficiency.
All my years working in the industry I would list as my # 1 item concerning efficiency would be the return air system as a whole. Just because a contractor has the correct amount of sq inches of return air openings for a given tonnage does not in itself make a balanced system. To me tthe most important is where the return air is taken from.
The least efficient return air is the one we refer to as a local return, the return air grill is directly above or directly below the furnace no duct work just a plenum. This type system does not allow for even distribution of air and there will always be hot/cold pocket within the home
The home I inspected today was just over 3300 SQ FT two systems one upstairs and one down stairs each system having its own thermostat. The Realtor had it listed as zone central air. This type installation is not a zone system its simply put two systems.
The downstairs foot print had 3 bedrooms ,1 full bath and one 1/2 bath on one end of the home. I will call it the south end, the south end had a hall with the return air grill in the ceiling. No other return grill down stairs the north end had the master bedroom/bathroom these bedrooms were seperated in the center of the home with the living rooms, kitchen and family room.
This is horrible set up the master bedroom stays hot in the summer cold in the winter no circulation except beneath the door. Most reasonable people except Linus sleeps with the bedroom door closed.
I snooped around the home until I found a easy way to install a return air in the master bedroom and informed the client that this would make the system more efficient and help with the hot/cold spots in the home. Did I put this on the sellers repair list absolutely not this was just info for the buyer to do with as he pleases. I will post a couple of pics from the attic above the first level the second level consisted of one large game room and a upstairs bathroom. The upstairs area had attic areas on all four sides of the room and also above the room, made it very easy to add additional return air ducts