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The King Kong 4000mAh cells are exactly what I use in both the L2 and in the L6.
I can’t say if the Fasttech ones are authentic but I have no reason to believe they aren’t. Fasttech is pretty good about that. Still, I buy mine directly from Richard at Mountain Electronics for right around the same price. Everything he sells is legit and it’s only a couple more dollars to ship them and you’ll have them much faster. I just checked and he still has 8 in stock. His KKs are done by EVVA and they do a very good job at adding button tops, protection circuits, etc. He also has the unprotected 5200s if you want longer runtime. That would be the same cell as the Keeppower without the protection circuit and they come in under 70mm. In the L6 you will not see any performance boost from a high drain cell with the stock driver being completely current controlled. He has several good cells for the L2 and L6 but yes I can personally speak for the King Kongs. Very solid performer and recommended by Richard himself.
Here’s a link http://www.mtnelectronics.com/index.php?route=product/product&path=59_108&product_id=189