Been thinking about this.
These lights are great, but.. don't waste your time modifying as much as I've been doing. Changing the LED to an XR-E R2 significantly improves the brightness of the light and IS worth doing if you are prepared to accept the reduced run-time.
What isn't worth doing is buying a different driver for it. It's much more hassle than it's worth and in reality I was more than happy with the light with just it running an XR-E R2, and an external power source
If it's gonna be used as a torch for bar mounting, I still think you might be able to squeeze in an 18650 battery to get about 2.5 times the run-time and that might be worth doing.
What it really comes down to is this: If you already have DECENT AAA rechargeables then these make sense, as decent 18650's are expensive and you still need to buy the charger etc. If you already have 18650's and charger then don't get these lights, as buying something else (brighter) will work out financially better.
IMHO, don't get these lights for the flashing function, you can get cheaper and better flashing unit's that last longer (as I've linked to in a previous post).
Hope this doesn't sound pessimistic, it isn't meant to lol. I really like these torches
EDIT: I wired up the 4x AA Battery pack tonight as well. Seems to be the same brightness as the XP4001 battery pack even though I have introduced about 1.3V more. Guess the constant current driver is doing it's thing, or I was already at the limit of the XR-E. One thing I did notice is that different lenses really change the spread.