With 3 of those lights in a 2'x4' closet your PPFD will be 786, now here's the math! 1 of your lights in a 18x18/2.25 sq.ft. area puts out 932PPFD which is 20.71 PAR watts per sq.ft. (932 divided by 45 =20.71) now multiply 20.71 PAR wattsx2.25 sq.ft and you get a total output of 46.6 PAR watts, that's your actual light output measured in watt's of each of your cob's not 78 as you said before. This makes your cob's roughly 52.4% efficient by the way... Ok so you have 3 cob's now so 3x46.6 PAR watts = 139.8 PAR watts covering a 2'x4' area or 8sq.ft so we divide 139.8 PAR watts by 8 sq.ft and we get 17.475 PAR watts per sq.ft, now to figure out PPFD we multiply 17.475 PAR watts by 45 (that's the PPFD formula in case your wondering where the hell 45 comes from) and we get roughly 786 PPFD, not 900-1000PPFD
Now let's compare it to a 600 watt HPS, the best HPS bulbs are up to 40% efficient, but lets say we only have a generic one that was already used for a grow, so we'll call it 30% efficient ok? And we'll cover the same area as your 8 sq.ft... So 30% of 600 watts gives us 180 PAR watts total. 180 PAR watts divided by 8 sq.ft = 22.5 PAR watts per sq.ft. and our PPFD formula tells us 22.5 PAR watts per sq.ft multiplied by 45 gives us roughly 1,012 PPFD. Now if it were a brand new top of the line HPS bulb never used we'd be closer to 1,350PPFD. You actually need close to 4 of your cob's to equal the 600 watt HPS, which is still great because you'd only be using 356 watts which means you'd only be using around 60% of the watts for the same PPFD.