the problem you have is a cumulative thing. A perfect system can tolerate high temps. However, a slight phosphate load, a slight nitrate load, a slight organic load, too low water flow, too low oxyegen will all add a little stress. Each alone is no problem. However each little part adds up.
I know on my system, if it runs any higher than 82 I see bleaching around coral bases of certain corals. Predominantly acropora nana. As soon as I correct the issue, they recover. If I dont....they slowly lose flesh.
The problem with heat is not the heat itself, its the decrease in the ability of the water to carry oxygen and carbon dioxide. As the temp goes up the amount of dissolved gasses go down. This can be further compounded by a large quantity of live stock.
from your description, sounds like you have a 75g with a single 250W....if you add another you will definitely need a chiller.
I ran two 250s over a 75g. I had no hood and two large fans switching on with the MH. The humidity was outrageous, the tank would lose 3-5g/day and i ran it at 80.
I would seriously consider a chiller. The benefits are enormous.
Also, looking at the corals you have listed, you may well be in danger of cooking the colt with too much light.