You have to consider that my 800 watts hps was more like 850+ watts (I had old ballasts) I like to overkill with my lights, well, if gram per watt was a real concern, I believe you can grow very decent pot using 30 watts per sq/ft or even less with LEDs... I think that this is how you would get the best gpw (gram per watt)
So if I was trying to get the highest gpw I would buy many smaller units and try to spread the light, especially with a SoG setup.
I have not grown long enough with LEDs to be certain of what I am saying ... so take it with a grain of salt.
Like I said though my goal is to grow dank and I want to grow big buds (I got some pretty big coming up!!! got a month to go and a few of them are HUGE) and I like to be 45-60 watts per sq/ft. The AF600s are very well built, very heavy and they use quality components. In a SoG setup I am sure the light would cover the full 16 sq.ft. but I have mine over about 12 sq.ft. ... but if gpw more important, 4 smaller units would most likely out perform 1 big unit ... I like the convenience of one unit too... and after I bought my 2 af-600 I started to look at the prices again and I realised that price per watts was almost the same from small units to the big (except the AF600 is a different series... a step above, but all other lights from fero/area51 are almost the same price per watts, big or small) Afterward I thought I should have tried 1 af600 and 2 A51-135 instead of the 2 AF600 ... Anyway, now that its all set and done I am glad I got the big guns