Join Date: Jun 2011
Location: Austin, Texas
Bikes: Unidentifiable CX-based franken-commuter
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Not all LEDs of one "model" are created equal. The "base model" you're discussing is the XM-L, but when CREE manufactures them some end up being relatively brighter and some dimmer. They separate these during post-production testing into "bins", T3, T4, T5, T6 for XM-L. There is also at present a U2 available which is about 7% brighter for the same current/voltage, but it has limited availability. A few high-end flashlights come with them, and you can get the emitter + board from Cutter Electronics, but I don't know of any bike lights with them.
Here's a reference for quickly comparing different LED's. They also have pages for other manufacturers.
The newly-released XT-E ad XB-D lights have lower current limits than the XM-L. While they are brighter at lower currents, if you're using a single LED you can get more light overall out of an XM-L. If you're using 2 or 3 LEDs, using these smaller, much cheaper LED's can be more cost- and power- effective, as well as enabling a more tightly focused beam. But at present, there's nothing in the immediate future to supersede the XM-L.