Hey Chromophore, I get the lights down to 6 inches from the tops or less. I have a fan blowing directly down the "gap" between the tops and bulbs, really helps.
Same with the HPS. I could bring that 150 down to 12 inches from the tops with my inlet fan hitting the gap.
Way cooler to run the T5's...but when summer hits I'm going to have cooling issues regardless.