Alright, I'll throw my 2 cents in as well.
1. How is your fan pointed? Ideally, you should have it pointing slightly down at the water creating some small waves and ripples rather than entirely parallel with the surface. The reason is that evaporative cooling is dependent on surface area. Your goal is to make your tank "sweat" more. You do this by disturbing the surface to create more surface area allowing more evaporation as opposed to simply attempting to blow hot air away. Aiming your powerheads such that you get more surface agitation also helps.
An additional benefit of more evaporation would be the necessity of having to add more (cool) top-up water.
2. Adding more water via a sump would definitely help since you'd have more water volume to absorb the heat and another surface with which to blow air across for further evaporative cooling.
3. That said, I don't think you have to panic about your temperature being ~80-81 degrees F? Many reefs are kepts in the low 80s.
http://www.reefkeeping.com/issues/2004-05/rhf/index.php
This is just one of a number of articles suggesting that such temperatures are fine for reefs.