Your guesses for sizes of heat pumps are about right for a poorly insulated house with reasonable (less than 10%) window area.
A well insulated house would have at least R60 ceiling, R30 walls, and R3 windows. My last house was insulated like that and had a one ton central air conditioner. In a stretch of hot weather where the daily highs were 100 to 104 deg F, the AC ran only 58% of the time. We normally heat to about 70 deg F and cool to about 78 deg F. It takes two or three days of 90 deg F or hotter to get the house from 70 to 78 deg F. A well insulated house is nice in an area where power outages happen.
Heating/cooling systems do not heat/cool by square feet floor area, but by heat loss or gain. The heat loss or gain is calculated from square feet of wall, window, and ceiling, plus internal heat gain. An example heat gain calculation for a 400 square foot corner bedroom on the top floor:
Wall area (20 + 20) X 9 feet high / R30 = 12.0
Ceiling 400 square feet / R60 = 6.7
Windows (3) at 3 X 4 feet / R3 = 12.0
Total = 30.7
Heat gain at 105 F outside 75 F inside = (105  75) X 30.7 = 920 BTUH
Heat gain from two people at 400 BTUH each = 800 BTUH
Heat gain from a big TV set at 100 watts = 340 BTUH
Total heat gain = 2060 BTUH. This is the load that should be used to size the AC.
Similar calculations apply to the other rooms and the entire house. If the heating and cooling systems are properly sized, they will keep the house at the desired temperature, and you will not even notice their operation. And the total electric cost for AC in hot weather will barely show on your electric bill, even at $0.23 per KwH.
Insulation gives you a more comfortable house, a house with low heating/cooling bills, and a house where the power can go off all day on a hot or cold day with minimal (about 3 deg F) effect on inside temperature.
Insulation also helps the entire house to be at the same temperature. My brother likes cold bedrooms. When he came to visit in winter at about 0 deg F outside, he blanked off the heat register, closed the bedroom door, and could only get the temperature down to 60 deg F.
