I've had a softie only tank spike to 90F for a few hours a day, and while the corals sure as hell let me know they didn't like it, they all survived it (I have since then moved the stuff ), I wouldn't say that's the norm though.
I run my tank at 80F. Most indo-pacific reefs are 80-84F year round. I have come home a few times to 82 and noone seemed to care. I have read that a few reefers actually run their tanks at 84 solid. 8O Seems a little high to me as well. I think that the occasional 84 spike in tanks is quite tolerable but prolonged events in certain areas cause bleaching in the wild.
I don't let mine get hotter than 84. I have a digital temp read out in a prominent place, and if it hits 80 I turn the halide off for the day.
This only tends to happen to me during spring and fall when it just isn't hot enough outside for me to justify closing the windows and turning on the AC, but the house is just warm enough for the tank to get too hot.
I had mine run at 84 for a long time. It wasn't a full reef, everything happens a bit faster at higher temps in my opinion, algae growth, fish diseases etc.
I keep it around 80 now.
Course its Airconditioned in here, my wife is freezing at the moment and I am hot in shorts and a Tee....
For along time I ran my reef with out a chiller with 2 MH so it was quite warm. My fluxuations would range from 73 in the am to 84+ in the PM. They only thing I had that had an issue with the heat was an anenome. Other than that everything was fine. Everything is accelerated when the temp is up as already mentioned. Since the chiller and controllers were added, 77 constant.
Yes I am roasting in the AC and my wife is freezing.
Look around locally for a used one. I picked mine up real cheap, 1/3 HP for 100. It was just refurbished by a refrige place and has brand new insides. Works great. Didn't break the bank and it will be good for many upgrades of tank size in the future.