Guys, thanks for all replies so far. Bob, there are 6 8" ducts and one
4". That sounds to me like about 1250 CFM, correct?
Some nfo which has now become more relevant...In the original home, 10
years ago, there were actually 4 ducts. When I bought I thought I would
need an extra supply in the main living space and asked for it in the
negotiations. The builder, blast his bones, added it without warning me
of the consequences.
Then, a couple of years ago, I added another 8" in the basement.

So it looks like perhaps the original 2 ton was slightly undersized but
not by much, while today, I just might actually need a 3 ton? Am I
reading the data right?