A huge +1 for an MC-E in the SF L2, there have been a few demonstrated here and they appear to be excellent. It should get 500+ lumens OTF if driven hard & well-heatsinked. If I was to get my SF L2 modded, it would go to 'Milky' for:
- MC-E emitter (I think that the existing reflector is reasonably appropriate for this LED)
- bored through to take 2x 17500 LiIon rechargeables (comparable in length to 3x CR123, yes the body should be long enough once the driver is removed from the body)
- installing a new 'buck' driver in the head appropriate for ~7.5Vin (The existing driver doesn't run at optimum with a single LiIon rechargeable, and is severely overdriven with two LiIons)
- changing the resistor in the tailcap to provide an appropriate low level
After this, the light would perform at optimum with 2x 17500 LiIon rechargeables (~7.4v)
and 3x CR123 primaries (~7.5v) for backup.
It would end up being a bit expensive, but would be pretty much a dream light IMO.