We could possibly do with a sticky information thread on the site if anybody feels expert enough to provide one.
My own research leads me to think that claimed lumen counts are very hard to define and in practice such exaggerations/lies are told that it's best to ignore them.
Cree XMLs are probably the brightest and best emitters currently available. The XML-T6 and XML-U2 are roughly the same brightness, but the U2 gives a slightly softer, less harsh light.
Nobody mentions wattages in their sales blurbs, perhaps because 10w doesn't sound very impressive, but an XML can be run at up to 10w and at that rate would be extremely bright - most applications run at lower amps and around 7-8w, which is still very bright. [I think if they were run at 10w for extended periods you would probably get overheating/loss of power issues.]
It seems that on the most powerful Fluxient you have a battery pack of four pairs of 2,200mAh 3.7V batteries, which equates to 8,800mAh 7.4V. I think the system runs at 1.2A with a 3 x XML-U2 light, so you get just over 8 watts per emitter and a battery life of 8.8Ah divided by 3.6A = c2.4 hours. The same principles can be applied to other systems to work out battery times etc.
If I understand it correctly, the system still runs at the same amps even when you go down to the medium setting, but LEDs work on micro pulses of power rather than a constant supply, so when you turn it down to medium you get the same amps but for half as many pulses per second (if it's a 100/50/30 system) - which doubles the battery life and roughly halves the light output.
I have a XML-T6 single emitter Fluxient zoom torch and it's very useful for off-road (wide beam) and road (narrow beam), but you can't beat more watts - three emitters is always going to give three times as much light as one of the same size. I personally prefer three emitters in one light over separate lights, as the emitters can be set up with the pitch of the reflector to give a broad integrated beam, whereas three separate lights will just send three separate beams.
Even though very cheap lights offer XML emitters, that doesn't make them good (still less reliable) lights. The emitter is only a fraction of the cost of a good set up, and the quality of the lamp housing, connections, batteries and battery charger are just as important - and these are the things that are skimped on in making the cheap lights.
I personally buy Fluxients off Jim Donaldson in Glasgow AKA torchythebatteryboy, AKA big_f_d_d on eBay. His prices aren't the cheapest, but I find him quick, reliable and trustworthy, which is more important to me. He deals mainly in diving lights and batteries, and you obviously have to have total reliability in that environment. He has a very interesting website on which he compares various different lights, batteries etc.
If anybody is able to add to this or correct anything I've said, that would be very helpful.