The most powerful mag LED mod list would be... messy and highly populated because of constantly changing bins. However, I would imagine Milkyspit has build a few of the most powerful ones with larger-than-stock heads. Essentially, the more space for emitters/reflectors/heatsinking you have the more powerful you can make it.
I think the 2nd-most common LED magmod (aside from Malkoff/EverLED/TerraLux drop-ins) would be a heatsink from KD or The Sandwich Shoppe for triple-stars and a trio of mini-reflectors. In that configuration you can get ~485 torch lumens if you're driving R2 emitters at approx. 1000mA (assuming your heatsinking can keep the junction temperature down, otherwise you'll get less). That would be... around (3 * 3.7V) * 1.0A = 11.1 watts, so driver losses would make drain from the batteries a bit more than that... let's say your driver is 90% efficient... that's around 12.33 watts. If your heatsinking is OK that puts you at 485TL/12.33W = 39.3 TL/W. Would be around 60.8 L/W from the emitters.
It might be of interest to note that if the Crees were pushed only a little harder we would be getting into territory that could be occupied by hotwires that use some of the new highly efficient
IRC bulbs. For reference, I belive the ROP tend to be somewhere in the 1000 BL/30W = 33.33 BL/W area if the appropriate resistance fixes are done.