If the indoor coil has more surface area, you might get more EER/SEER out of it but when cooling that extra surface area can diminish some of its dehumidification capability because the coil needs to be saturated before the water starts to run off of it and once you've met the temperature demand of the cycle and it shuts off, all of the water remaining in the coil will evaporate back into the air after the cycle and be return to the building. Taking a look at the AHRI ratings for 1.5 ton and 2 ton units of the Bryant variety in their 16 SEER models, I'm usually only seeing a boost of about 1/2 SEER by jumping from a 2 ton to a 3 ton coil. I'd rather sacrifice a partial SEER point and have better dehumidification and get a condenser and blower that are as efficient as they can be if I'm going with a central air unit because reducing latent heat(removing moisture) is very important, it isn't just about removing sensible heat(reducing temperature).
I currently have a 2 ton and if I went with a whole house heat pump I'd be willing to keep the slight oversize because I can manage to run a single cycle and still be plenty comfortable but a 1.5 ton would cause the temperature to be more constant while its running rather than have it get to 75 to 78 before powering on and have it shut off at 70-72 when it is done for the day. I've found that this approach removes enough moisture in the air to make the next day's high 70's feel very comfortable. I'd only go to a 2 ton for if I wanted a heat pump because I'd get closer to the capacity I'd need for heating without too much cooling oversize for my house, also the HSPF and EER/SEER usually is higher in the 2 ton units for the brand I'm looking at because people who have low loads to use a 1.5 ton aren't usually going to use too much energy in comparison to someone who needs a 3 ton, for example, and it's harder to sell premium efficiency units in that size.
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