Want to provide a data point, results of advice received here 2 years ago.
A couple of years ago I first came to this site to learn what I should look for as I was replacing my old A/C that died. Learned a lot.
One of the main topics was whether I could downsize from a 3 ton unit to a 2 ton unit in my 40 year old 1800 sf house in Charlotte, NC, since I had better insulated. Opinions varied.
My personal Manual J done with the software on this site said 2 tons should work. Others said I should only downsize to 2.5 tons. But since I wanted two stages, I decided to risk it and go down to 2 tons.
Last summer was not all that hot, but the past two days have tied the all-time record for Charlotte, and the two tons have kept up fine. I only set my A/C at 76--below that and I feel chilled--but it runs on low stage most of the time. Yesterday afternoon, to test the system, I lowered the setting to 75, and that made it kick into high, but it brought the temp down that extra degree in less than an hour with outside temps over 100.
My humidity stays right at 47-48 percent all summer, the unit is quiet and cooling is uniform. When the humidity got down, my son's dust mite allergy went away, no more morning sneezing. Electric bill is been a good bit lower, too.
So I got the right result, thanks to all who helped.
Lessons learned, insulate, airseal attic, and lower size of equipment. My ductwork was not bad, but lowering size of needed equipment will compensate for the often undersized ductwork ya'll say is rampant.