saying that a bulb is 6V has absolutely nothing to do with how overdriven it will be at 8.4V unless you also list a life rating at 6V. The WF-500 lamp is ALSO a 6V lamp with a particular life. FYI, it's also a 2V space heater and a 3V glow toy. Just depends on what voltage and life the manufacture would like to market the lamp as.
Philips specced the lamp for 6V usage with a certain rated life. I have no idea what the published life rating is of the lamp at 6V.... Most 6V bi-pin halogen style lamps are medical style lamps rated for 100 hours at 6V. I'm betting the lamp used in the solarforce jobs is a 6V 20W 100 hour lamp. Or maybe possibly a 6V 15W 100 hour.... not sure... maybe if someone owns one they could take a tailcap reading on amps so we have a better idea what it is...
Take a moment and consider some of the "mod" lights people make around here. for example, the philips 5761 on a pair of li-ions, which is also a 6V 100 hour lamp, take a look at the ROP, whose bulb was originally developed for use on a 6V battery source (lead acid 6V), and gets used by CPFers in 6xNIMH cell and 2xli-ion cell configurations all the time.
Now, you have to realize, that no 18650 li-ion actually delivers 4.2V into a load, and every flashlight has resistance. Every point of contact, between the cells, the springs, the switch, the bulb springs, the contacts to the pins of the bulb, etc etc etc, all have resistance that will reduce the actual voltage at the lamp. If you go over to Silverfoxes li-ion shootout, we see some examples of 18650 sized cells operating at 3amp, which is the general range we can expect this light to be in, we see that best case scenario, we get 3.75V at the cell fresh off the charger into a 3 amp load, so I would estimate, that on a perfectly maintained flashlight, with the highest quality cells on the market, you might, and I emphasize MIGHT, be able to get 7.4V at the bulb fresh off the charger, by my calculations, that would make for a very bright, very efficient output with about a 8-10 hour bulb life rating, keeping in mind, that every moment after that moment, voltage is dropping further. towards the end of the discharge, the lamp is being driven in the 30-50 hour life range, so you average it all out and I expect that philips lamp to deliver somewhere around 15-25 hours of service life in that configuration. Possibly more considering that getting 7.4V at the bulb might only happen on the first few cycles, after which some oxidation sets in and brings that number down a few tenths.