Wow. I dunno, but I want to. Subbing this in case someone out there smarter than me can break it down for us! Overall, if heat is an issue, get a cooled hood IMHO.
First time I ran a 1k was in a walk in closet with no air cooling. That closet was in the 90's with the door open, them bitches will get hot if you let them!
Lets say that ambient temperature is 75 degrees. A 400w light may run at 100 degrees. 25 degrees above ambient. A 1000w light may run at 137.50 which is 67.5 degrees above ambient temperature.
I made these numbers up, but you see what I'm getting at. The bulb doesn't have to be twice as hot to give you twice the problem.