In a 1x123 form factor, the KX2C needs a 3.7V RCR123 (rechargeable). I have an single mode E2DL head on an E1E body, which is basically the setup you're describing. In my experience, output is just slightly less than the same head powered by two CR123 cells, but more than a stock E1B. I would guess about 90 lumens. However, another member posted that output was slightly less than an E1B, so your mileage may vary. I can't remember what kind of runtimes I was getting. I think it was about 45 minutes of regulated output.