Is your 6P the older plastic lens? All the newer production lights are glass. Light output won't change noticeably, if at all, between the two. I don't think you'll be able to tell the difference with your naked eye. And if you insist on glass, just pick up a new Surefire bezel on B/S/T for cheap.
You can check out the different with lens (window) and no lens (no window) by rermoving the head of the SF and pressing down on the M60 module so that the module makes contact with the rim of the body and turns on. Press down with two fingers on the side of the drop in. Compare the beam to the completely put together SF with drop in. Do you see any diffference. I do not believe you will see any difference, as difference will probably be less than 5%, if that. If lens (window) is plastic then difference might be closer to 10%.
