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It will probably be more like 500 lumens or less, with an 18650 and about 300 lumens or less with 3xAAA. Don't believe the lumens estimates as a general rule. Instead, do some more looking here, for output of emitters. Like Here, here and here. A single XM-L will not ever come close to 800 lumens in that type of light and much less with a stock driver.
As a general rule, anything powered with a choice of Li-ion cells over NiMH cells, will always be brighter with the Li-ion cell. You can expect to be fairly unimpressed with that light overall, but that's part of the learning process.
If you have AA cells and want a bright light, then you might want to save up for one of the lights that takes 3xAA or 4xAA cells. There are a few that do.