Per the technicians I spoke to at Surefire. The eye will not be able to visually discern between a 500 and 600 lumen version UNLESS they are side by side. Even then, it's tough. I tested that statement in both normal and low light conditions and agree with the tech.'s statement but like others have said, Mo' lumens is mo' better if you have the option of course.
However, If for some reason you have to send it in for repair they will upgrade it to the latest internals (600 lumen). I would ASSUME that there's an additional cost associated with said internal update / upgrade, but we all know what happens when we assume things.
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