It has been my experience that the actual temp is not as much of an issue as how fast the temp changes and how often it changes. I don't have a lot of experience with Anemones, but with Fish/Reef - They can cope with higher temps, but the Temp Swings are deadly.
If you tank got to 84 over an extensive period of time, I would imagine that your anemone would not tweek to hard. But if the Temp got there in a short period of time (say 12 hrs.), I would imagine it would hate it. Especially if the temp drops at night and heads back up during the next day.
IMO
-Rich