Your cooling issues may not be size related, but rather involved with inadequate/leaking ductwork and/or insufficient return air. Make sure they check this first before upsizing. If they upsize and your ductwork can't handle it, just asking for more noise and more comfort and efficiency issues. Based on square footage alone it sounds like their proposal is oversized. Did they do a formal load calculation?

Don't know if a 'formal' load calc was performed, but I questioned the sizing and was told those were the correct numbers for my house. As for the ductwork, every room has its own return and although upstairs and downstairs returns share a common return trunk in the basement on the way to individual plenums, the contractor said it was adequate. As I mentioned, the old 24,000 ACs couldn't quite keep up on those 95+ days, so perhaps additional BTU's are necessary.

Going from 4 ton to 5.5 ton on 2500 sq ft in Jersey seems excessive. Around here, and we have similar heat in the summer, we'd probably have 1 system and 3 tons, 3.5 tons max. I find it hard to believe the house has a duct system sufficient to upsize the equipment like that! Most systems are starved for airflow as it is.

I don't see going 2 to 3 downstairs. Cool air from above falls and warm air down goes up. Maybe the 2.5 ton upstairs makes sense.

Oversized can mean humidity control issues. I wouldn't do anything without an accurate load calc being done. Then someone should check the static pressures of the duct systems and be sure there is the capability of moving enough air. You could spend all this money and find you aren't any more comfy than before.

You were provided excellent advice.

I would want more insulation in the attic, especially for the cold winters.

Your winter design is around +10-F, while here in WI mine is -11-F both at 99%.

The worse thing you can do is oversize the air conditioning. It will greatly lower the SEER Rating & be very inefficient. - udarrell

Here is a link that might be useful: Sizing