I think I would appreciate a 1 or 2 degree Celsius drop in temps.
I'm in a smallish cabinet. Almost any measurable decrease in cab temps is good for me. But mainly, I'm thinking of adding a second bulb to my current 250w HPS (it's a 250w MH for veg). With my current cooling arrangement (SuperSun 2 hood with 6" air cooling) I might be able to add a 250w CMH and still be ok temp-wise. Or, I do have additional cooling capacity partially installed in my cab. I could possibly add a 400w CMH to go along with my 250w HPS if I finished the cooling upgrades.
With the current cooling, I'd be afraid to try doubling my 250w HPS and still keep the spread between the temp outside the cabinet and the canopy within 2-4 degrees F. If the spread gets more than that I have to start running my central A/C more than I normally would. If sleeper's CMH lamps are cooler enough to create about 3 deg F difference in his room temps, it would not surprise me if the difference could really matter in my 6 square foot, 30 cubic foot cab [shrug].
I try and mostly succeed at keeping my lights-on canopy temps at or below 80F. Granted, 80F is a somewhat arbitrary goal. Nonetheless, I care about the last few degrees. If only I could borrow a few different ballasts/lamps for a few days and try out different arrangements in my space. I'd test the hell out of everything until figured out what the difference would really be for my particular setup. I find it difficult to apply others' experience and impressions about temperature to MY setup. There are so many variables.