As a practical matter ,it's more an issue of the ranges of temps the fish and corals are used to than an absolute high temperature they can tolerate. Idiosyncracies for each aquarium ,such as :nutrients, bacteria, flow, bioload, etc. make it impossible to predict a reaction to temp variation with any certainty.
For corals certain clades of zooxanthelae which they can acquire are more/less tolerant of higher temperatures. We don't know which ones our corals may have.
For fish ,pathogens and parasites that may be present can get a boost from higher temps,bacterial activity throughout the system will likely increase and the fish may also suffer from O2 depletion at a time when their increased metabolic rate demands more .
Some deliberately acclimate corals and fish to broader temp ranges. FWIW, I just keep mine a pretty steady 77/78 with peaks to 80 from time to time.
O2 is less soluble at higher temps and I'm always concerned about nightime hypoxia in my overstuffed mixed reef tanks. . Increased metabolic rates at higher temps increase O2 consumption by the organisms in the tank and the waste they produce as well.
I would watch the fish for signs of parasites . I don't think 84 is terrible but it's hard to say that with any certainty as a general guide.
My tanks were there yesterday afternoon, and everything is great!!
