The UltraFire batteries are notorious for overstating their runtime. By a lot. Tests on CPF have shown that their 2400mAh batteries often don't break the 4-digit barrier. Currently the best available 18650s are the Panasonic 3400 series, which are sold in protected form by various manufacturers (EagleTac and Orbtronic, and I think even Int'l Outdoor has some). The other bonus is that the reputable 18650s also have a much longer cycle life, so not only do they have higher capacity they will also keep that capacity through many more charge-recharge cycles. Just make sure your charger is up to the task too; the cheap chargers are known for overcharging batteries/not shutting off. Get a multimeter/voltmeter and make sure the batteries leave the charging cradle when they are at 4.20V. Don't leave them unattended.

Also, 600 lumens from a single XM-L is in the territory where heat buildup starts to be the overriding issue. If your 3xAAA/weak 18650 batteries were holding you back before, make sure to check (touch) the light once you switch to the better 18650s, you may find that the light-head gets very hot very fast. Usually not a significant issue so long as you keep air moving over it, but if the LED itself is not well heat-sinked, you may find its lifespan dramatically reduced.