The truth, in my experience, is that indoors the strains I have grown seem to tolerate temperatures in the 80's, up to about 88 degrees in the canopy and warmer for brief periods of time. Temperatures in the
upper 80's are not favorable for ideal growth; certainly not for prolonged periods. AFAIK, at temperatures beginning to exceed 86 degrees the stomata, the pores on the leaves through which gas exchange occurs, start to close up. If the plant isn't exchanging gases (oxygen, CO2) then it isn't photosynthesizing. Stoma need to be open in order to get CO2 and while they are open, water is being lost (evaporating). So in order to conserve water in the heat the stomata close up. Supplementing carbon dioxide opens stomata to compensate for higher temperatures.
Otherwise circulation is extremely important to facilitate gas exchange and dispose of heat. Summer time is the worst for growing indoors. I would suggest considering air-cooling for virtually any HID lighting. It is simply more efficient to remove as much heat as possible from the source with a fan that uses maybe 150 watts versus a thousand or more. You still may need to use an AC with cooled reflectors, especially with multiple lights. My two reflectors are sealed, and air cooled with a 6" HO ventilation fan (with carbon filter) that is drawing air from inside the tent. There are 2x600 watt HIDs, still need to use the air conditioner in the summer, with temperatures between 78-87 with lights on lately. I have a Kandy Kush under one of the lights and it seems to be doing just fine, with buds growing about 5" from the lamp.
So please tell me then, what the difference is between the temps being to high indoors but not outdoors if the conditions are consistent? I'm confused.
Outside plants have access to an open atmosphere of carbon dioxide. It is still likely their stomata are closing up during peak temperatures, assuming they are in full sun in temperatures over 90f.