The SST-40 is a super-high power output 3V nominal LED. If your goal is maximum output from a 3V LED without concern for overall efficiency or heat, then this is the one you're looking for! Capable of 2,000+ lumens direct-drive with an FET driver @ 7-8A.
Low Forward Voltage Current Warning!
Due to the extremely low forward voltage of this LED, some care needs to be taken when used with a high drain 18650 or most 26650 or larger cells: even with perfect heatsinking (better than what you can get in most flashlights), the LED will usually die at around 9.5A, which can be exceeded with a good battery and low resistance setup. It is recommended to run the LED at around 8A-8.5A maximum continuous, even with good heatsinking. This means that with some builds you may have to add some resistance to your system, with thinner and longer wires, less efficient bypasses, etc.
The die size is similar to the XM-L2, as is the overall beam pattern. For more information regarding potential output, check out djozz's test results.