2 unit house (seperate-ish for upstairs/downstairs).

Was 2.5 ton and 3 ton. They are both 3 ton now (2700 square foot house)

I think our cooling problems may be tied to something other than the unit.

Several things to note..

Upstairs differential is hanging around 8 or 9 degrees (while most of downstairs is closer to that 15-16 degree range). (this was measured with a point and shoot thermometer which is apparently frowned upon) It cannot keep up with a 92 degree day here in Alabama (which was a problem before as well). It crept up to 80 on Saturday and it took about 9 hours to get it back down to 76 (with 5 of those hours being after dark).

I wish I would have checked before we had the unit replaced, because I'm guessing the differential was still around that much pointing towards (maybe?) a ductwork issue?

I think one of our issues is the stupid 20+ foot ceiling in the living room that opens to the second story "hallway/catwalk" which is where the second story return is.

Another issue is (probably) that there aren't any supplies in that entire area. A good 150 ish sq. foot range of hallway in the hottest part of the house. Another issue is probably that the entire living room downstairs is supposed to be cooled by 2 vents in the top of the ceiling (20 feet up). Which is fed by the downstairs unit.. So that is two supplies fighting to get through 20 feet of accumulated heat to get to occupants (300 ish square feet, plus that upstairs hallway/catwalk area). There are 3 supplies in the master bedroom (which is a much smaller area). As best I can tell, those supplies are around the 8-9 differential (and they come from the downstairs unit) but they do not run through the walls to get down to the actual downstairs. (meaning they're coming straight out of the attic). The supplies that are actually downstairs (kitchen, dining room, guest room, and bathroom) are where I'm getting that nice 16 degree range.

Also the upstairs rooms on the south side of the house are extremely tough to cool. Typically they run about 5 degrees warmer than the upstairs thermostat. Luckily we have two Ecobee thermostats now with additional sensors so we can actually cool those to what we want them. I pointed my laser thermometer at the ceiling in there and it was ranging around 85. Whereas everywhere else it was closer to 78 or so..

Also of note.. The thermostats are both consistently saying that we're running over 60 percent humidity. Usually around 62. I bought a hygrometer today to see if that is correct. But that seems high, right?

I have Window ACs downstairs in the main room, master bedroom upstairs, and guest room upstairs (the REALLY hot one). I usually need to run these to supplement if i want the house to be below 78 on a hot day. I have to run the one in the upstairs guest constantly if I want to keep that room anywhere near livable.

So I know this is a long post with a lot of stuff to delve through. My wife's dad did the new install and he's coming back to run some more diagnostics which may point to a culprit, but I'm wondering if anything sticks out in the meantime?

I think there may be a problem with the upstairs ductwork. I think the vents were also placed/designed very poorly. I think there may be an insulation problem or something going on on the south side of the house where those rooms just get so hot.

Thinking of having an energy auditor or someone like that to come out and look. And a ductwork person. Just wondering if these seem to be good idea?