um... The bulb in the WF-500, R500, or any other flashlight, is no specific voltage bulb, saying a "overdriven 6V bulb" without mentioning the life and lamp type at 6V is meaningless as to whether or not the overdrive is a problem or not.
A WF-500 bulb is also a 6V bulb if you want to call it that, but has a 250+ hour life at 6V instead of ~20 @ ~7.4V....
As long as the philips lamp is rated for a decent life at 6V (like 250-500 hours) then it will work fine on 2x li-ion just as well as the WF-500 lamp.
To be honest, most common halogen lamps on the market are rated for about 2000 hours, so I would be less worried about bulb life on 2x-li-ion, and more worried about it being awfully dim because it's not driven hard enough.... but I'm sure the lamp is picked out to work correctly....