Ah, you're starting to ask questions that I'm still learning about, hahah. You are correct that different flashlights are driven differently. Some are overdriven at the expense of heat and lifespan. The drivers can be very simple or very sophisticated. An example of a driver can be a simple resister in series with the LED or it can be a pulse modulated IC to provide varying output. The varying sophistication of the drivers can also dictate the efficiency of the circuit. A lot of the drivers will regulate the current to the LED to maintain the brightness level despite the dropping output of the battery.

Speaking of drivers, I came up with a simple driver for my LED rocklights based on some articles I found on the web. Basically these are the same things as the roundeyes LEDs but at a 1/8 of the cost. web page

I didn't see on the SF website where it said the U2 output varies from 80-100 lumens. I doubt you'll get a bad LED from SF. Luxeon categorizes their LEDs into Bins according to color and output quality. I think SF would only get the best.

You might want to look into HDS lights, they're pretty popular among the CPF crowd.