Sorry for the cross post but i need suggestions ASAP. We are in central SC. The house is a 1.5 story with approximately 4400 sq ft. The upstairs has 3 bedrooms and a small open area as well as a long playroom over a 3 car garage. We are going to foam the roofline and upgrade to R-15 in the walls. There are two units (13 Seer). The downstairs is gaspac and upstairs is electric. Our builder has offered the following now that we're foaming and a lesser unit is needed.
1- take a $1,000 credit
2- upgrade downstairs unit to 14 SEER, variable speed unit with dehumidifier.
3- leave units as 13 SEER but make the room over the garage a separate zone.

We could do a combination of these but would have to pay additional (about $1,000 to zone the room over the garage separtly, and about $1,000 to upgrade to the next SEER level). so I want to make sure we will get a good return on the investment.