A light draws n watts from the wall. Less efficient epi-whatever lights can cover n sq ft at 50w/sq ft. That's their efficiency. It actually may be more. One person here is considering running over 70. I was looking at a grow last night that was considered "wall of fame" material at 90.
The DIY cob guys are doing 25'ish w/sq ft. The Area 51 is spec'ed for 26, if I recall.
If your goal is to reduce power consumption and heat, more could definitely hurt. Especially when you have run *a lot* more like the epi-whatever lights. In that case, T5HO produces more light, less heat for short grows. CMH for tall.
If you get into the COBs described here, more doesn't hurt as much. Going from 26w/sq ft to a gargantuan 35 is still less than what MarsHydro specs their lights at (yet seemsto have to run them at 2 to 3x more. That's probably what you would encounter with that cob-like object you were looking at.).
So, it's kind of relative. You can definitely get away with "more" using efficient LED without nearly as much heat as you would have with epi-whatever lights.