75F is ideal, 85F is max, anything above 85 and *MOST* plants growth will start to slow down. Above 95F and most plants will start to cook..
I peak at 84F.. but i used to have a seperate box that would peak at 90-92F and it was very noticeable that the growth was slower in that box.
As someone else said though some tropicals love it.. my thai super skunk went crazy despite the 92F... 0.o