On my old tank I was able to keep the temp in check using a fan and an Aquacontroler Jr.. I used the AC jr to turn the fan on, and turn off the pump on my skimmer, and my MHs as the temp increased.
It is not so much the high temp, as the overall change in temp. If you keep your temp at 78 and the temp goes to 86 you could have problems. On the other hand, if you keep your temp at 82 and it goes to 86 you will probably be ok. Temp readings on tropical reefs have shown an average temp of about 82 degrees, with fluctuations going much higher.
Some people, keep there tanks at 82 in the summer to avoid a large swing.
I choose to go with a chiller on my new tank for a number of reasons. I don't want to increase the humidity any further in my Fish room, so fans are out, and I don't have an AC in that part of the house. The room is also situated, so that it is very easy for me to vent the heat from the chiller outside.