The above link contains alot of info on efficiency and some lumen/watt efficiency ratings. Otherwise, I would suggest looking at this: Cree Product Characterization Tool
http://pct.cree.com/

I halide bulb that I feel would convert closely from the lumen scale to the PPFD (photometric to radiometric) would be the Ushio 10,000/14,000K bulbs. 250 watt versions of those bulbs come in at 10,500-11,000 lm. The 400′s come in at 18,500. Looking at just XP-G’s alone, in CW, assuming R5 binned LED’s (I havent seen S2′s available), it would take at least 55 of the XP-G’s @ 181 watts to match the output of the 400watter (103.7 l/w… 18,750lm). With 7 LED’s per PCB, thats pretty much 8 of them going at just under $50 each just for the LED’s. The optics can manipulate the light better than a halide reflector, possibly allowing you to match the output of a 400 watter with about 20-25% less LED’s IF a narrower spread is okay. In cases where such a focused output doesnt help (like over a shallower/wider tank) then this doesnt provide any advantage over a halide+reflector.

What hinders the efficiency of a system like this AI is that while there are 16 XP-G’s (for 5454 lm), the XP-E blue LED’s are no where near as efficient, nor as strong in output. Luckily, the XP-G’s have a strong blue spike in their output, but the the XP-E’s could still be seen as an efficiency dump for the whole system. If there were just XP-G’s (24 of them), you would have 103.7 l/w. Now, I know that the lumen scale doesnt favor blue, but lets say the output of the XP-E’s was in green to weigh their output more like in the radiometric scale (or we could say that we would take a lumen scale rating where all colors are weighted equal, bending the rules, I know), then the effiency drops to something like 80 lumens/watt at best.