If you experience a temp spike the best thing you can do is drop the temp back into the normal range as quickly as possible. There is no evidence whatsoever that temperature swings are stressful to anything we keep, whereas temperature spikes definitely are. You want to drop the temp quickly because temperature stress is dependent on the time and magnitude of exposure. The farther the temp gets above the tank's normal maximum and the longer it takes to return to normal, the worse the stress is for the animals. As Rob pointed out, you would be hard-pressed to emulate the speed of changes these animals experience in the wild where temps can drop several degrees in a matter of seconds.
Also, FWIW 84 isn't a bad temperature to keep reef animals. There are only 2 real concerns. The biggest concern is whether the tank usually maxes out at a colder temperature and the temp spiked at 84 today or whether the maximum temperature has been creeping up over the course of a few weeks. If it's the former, then the fact that the tank is at 84 is a real issue because the animals will not have been acclimatized to that temperature. Anything more than 2-4 degrees above the maximum they have been acclimatized to will cause them stress. Such a spike also tells you that you may have malfunctioning equipment. However, if the max has been slowly creeping up over a long period, the animals have had time to acclimatize to that change and will be absolutely fine at 84.
The second concern is that if the tank is reaching 84 at this time of year, it may get beyond 86 (which is about as warm as you can go and still allow for a safe margin of error) by the middle of summer.
