Mowood I have heard the same thing from my Bro. A guy he know's is getting 2lb a plant or a bit more, but his quality is not up there with my bro's but that aint to say that DJM's isnt the bomb or the next guy's.
you can get large plants like that and still maintain quality..but for large plants you NEED high rh..as they require more moisture to stay hydrated than a small plant...i use to run 1 plant per light for many years and usually got 2 pounds or more...quality is the same as if i ran 16 per light
here is a pic of an old grow running 1 per light...plant on the left did 2.25 pounds plant on the right did 2.5...hoods are 4 feet wide as a reference point...strain is chem 4