The "ideal" rule of thumb most people hear is 20 degrees cooler than the air entering the system. Without having several idicators, including the break down of the particular models ARI ratings, this is hard to determine. What is refered to as the Delta T, or difference in the split, is directly related to the dry bulb temperature but more so the wet bulb temperature. So, the higher the wet bulb which refers to how much moisture is in a cubic foot of air the less the temperature difference is going to be. On a super high efficiency unit, if you say have 80 degrees going into the system with a high relative humidity,(over 71 RH is out of the manufacturers design), then you may only have a 5 degree drop. But if you have 80 degrees entering with a low RH, you may have a 27 degree drop in temperature. This has to do with how much energy it takes to change the state of moisture. It takes 1 BTU to change 1 pound of water 1 degree, but to change the state of moisture , ie 32 degree ice to 32 degrees water (no temperature change!) takes 323 BTU's for that same pound of water. Moisture levels in your home are what rob your system of capacity.