I might be crazy here BUT. I am not sure that Vf has that much to do with the fact that your kl4's run brighter.
I don't think I have seen any LUX V bind which are as low as the 3.5 to 4 volts that would be L in terms of voltage.
I seem to remember that ALL of the LUX V emitters had somewhat higher Vfs.
I also have a few of these KL4's (love"em) I think the fact that they respond to the 3.6 volts of a rcr123 has more to do with the driver boards dealing with the low voltage better. going into a semi Regulation.
Again I am possibly wrong here.
One of the strange things about the ability of the kl4 working on 3.6 volts was/is the randomness of it. Some worked great some did not. Surefire must go these emitters in reels. Why would there be such wide variations. I found this to be true with the tints as well. I spent a year attempting to match 2 kl4's what an ordeal.
The little light shown above a few posts looks trick. The Black Kl4 is really beautiful. I have so many natural KL4's not a single black one.
The E1B is a great little flashlight. I traded away my L1 flashlight for an A2 recently. I replaced the l1 with a e1B and haven't looked back for one second. Anyway you look at it Surefire makes some damn cool lights.
I love all of my kl4's too.
Oh the horror when will it stop?
Yaesumofo
Not every KL4 head had the ability to use a 1x123 primary. I've been fortunate enough where all of mine are very bright and has a low enough Vf to handle 1x123 primaries.
Using rechargeables - no issue at all!
-Steve