My screen for the scrog is 4 feet by 2 and half. The lights cover a bigger area, but they start to drop off a tiny bit at the edges. I probably should have used reflectors, and like Michael Huntherz suggests, dimmable would have been smart too. But I started basic for my first build. I keep my lights about 24" up for veg, and bring it up to 36" for a short time after I pot up to reduce the stress, but planning on being around 12" for flowering.
I run them using MW-60-1400 drivers (60v, 1.4amp) one driver per COB. There are drivers that will run multiple COBs, but my thinking was if a driver goes, it just impacts a single lamp this way. Costlier, but more maintainable, and since the lamps are rated for 50,000 hours (about 11 years of constant growing), I figured that maybe could be a possibility. So I drive them at about half what they are rated for, I think. I'm not smart enough to tell you the exact efficiency though.
And yeah, they stay cool. I just use my garage, not even a tent, and they warm up the canopy maybe 1-2 degrees over the rest of the room, grow area has been a constant 72-77 Fahrenheit, The lamps themselves maybe get to 80-85ish? The CPU coolers they are attached to seem cool to the touch, even adjacent, so it seems like it's under 96, by touch.
CXB3590s that Michael mentions are top of the line, last I found some, they were about $80 a pop, that I could find. The Z2 cxa3070s I used were $23 from Mouser. I went with cheaper for my first build, as it was as much proof of concept for me as anything else, so wanted to go inexpensive. But now that I know I can do it,. I have my eye on them.
And what he says about picking the voltage you run them at is one of the key points in designing. Higher voltages = more heat, more light, less efficiency, more degradation on COB potentially, need fewer cobs for specific area vs. lower voltage = less heat, less light per cob, more efficiency, less degradation, need more cobs. Ideally you'd run them at as low voltage as possible, but that's balanced against cost of more cobs.