I contacted the folks at Underwater Kinetics a couple of months ago about the SL4 and SL6 lights with LEDs as light sources. I don't have the return message any more but they will be introducing the SL4 with an LED this year, unfortunately I can't remember when. At this time the appear not to have any plans for the SL6.
I was looking for the light as a backup light for cave diving, so I was interested in long burn times. The direction that they seem to be going is slightly longer burn times than the incandescent version, but not dramatically longer burn times. The light was represented as being fairly bright with a high color temperature. Since the light did not seem to fit my criteria as a backup in a cave I deleted their message to me and moved along.
Hope this helps,
Mark Vlahos