Here is a problem you are going to run into... an overdriven halogen is more efficient than an overdriven LED... the highest efficiency LEDs are more efficient than over driven incan.. only when not over-driven... which means.. to get more light (runtime) and the same amount of light you need a bazillion (maybe with 15-20) emitters..
If you really want more runtime... get your efficiency maximized.. add another cell to your pack to go from 14.4V to 18.0V..
some examples.. an osram 20W 1149 (model) lamp run from a 14.4V source direct-drive with an estimate 100mohm resistance.. (for switch and wires).. will get 14.22V to the lamp and get you 568 bulb lumen and 369 torch lumen... that is a mere 21.8Lumen/watt.. and say you have a 2.2AH (presuming 18650 cells).. you'd get 1.2hours runtime..
take the battery pack and add one more cell to get 18.0V nominal.. and run an osram IRC lamp.. take the smallest lamp they have.. 25W model.. and you get 17.74V to the lamp and the output is 1436/933 lumen.. with 30.7 lumen/watt! (40% more efficient).. now this is a lot more power, so your runtime would be lower, so it's not the best solution for your cause..
however...
you mentioned using 2x20W.. if that is the 12V rating than at 14.4 you are pushing 26W each or 52W.. you'll also be getting a net output of maybe 738 total lumen.. whereas.. running a SINGLE '25W' IRC lamp from 18V the energy cost is 46.7W and you get 933L.. so.. 26% more light and longer runtime..
if you want to have extended runtime.. you could have a low-output lamp in one of your headlamps and a high-output in the other... however.. it's very tricky to find lamps that will run at the same votlages and include the ludicrous efficiencies of the IRC lamps... i'll note.. that the 25W IRC has only about the same efficiency as some normal lamps.. so let's look at another solution:
the osram 64610 lamp.. runs very well from a 14.4V source if you regulate the output to 13.0V.. The numbers look like this:
62W.. 4.77A.. 1376 TORCH lumen... it's a knockout!... but there are other lamps that will run nicely from a 14.4V source and still have a high efficiency..
the 1166 lamp.. i run it at 12.5V.. 35.6Lumen/watt.. 2A.. 594 Lumen each!
The 1154 you can direct-drive from 14.4.. 883 lumen and 3.38A..
maybe.. a better solution is making your pack 3S2P.. adding two cells but dropping the voltage to 10.8...
you can then use the classic 1185 lamp.. efficiency is 34 L/W at 10.8V! it is pulling 36W at that level.. and you can use the lower-energy-cost 1331 as a swapper at taht voltage dropping the draw to 22W/lamp but efficiency is sky-high of 36.3 L/W.. that means.. simply by swapping to 1331 lamps and using a regulator to hold back voltage spikes.. you can get your output for two lamps up to 1048 Lumen.
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ok.. that's my 'incan will kick the snot out of led' rant.. now i'll go the LED route since that was your question..
To get nearly the same ouput.. aprox 738 lumen.. you will need at least 7 powerful emitters.. and since it's not terribly difficult to host 4 into a mag-head you cold take two mag heads and use a driver for each group like fatman.. you will get maybe 8000-10000 lux each head... that will only compare to maybe 15000-20000 lux your current lamps provide (as mentioned before .. the LED solution will absolutely be more flood and less throw)..
The other option.. is to use K2 emitters which are higher voltage.. it's possible to use an LDO driver to drop some of the 14.4V down to the 13.2 or so required for maximum output.. you might get 'enough' light from six K2 emitters.
To solve the throw problem.. you can use a bigger head... and mix n match some reflectors.. for example.. putting 3 27mm reflectors into a light.. and 2 of those.. you should get your output up to maybe 10,000 lux per head with only 6 emitters vs 8...
The interesting thing though.. if you run 6xK2 at full power you are only talking about 40W total.. it might be what you are looking for, especially since you can DIM LEDs! you don't loose efficiency like with incan when you dim.. if you dim an incan to 50% power you get 20% light.. if you dim LED to 50% power you get 50% light.
another tip.. use some 'greenish' tint emitters like V1 tint.. you'll thank me later..
http://tv1j.rouse.com talks about that magical outdoor tint.
There are benefits to each.. but with high-power leds at about 25 Lumen/Watt and high-end-high-power incan at 35-40+ lumen/watt.. if you want more light from your battery.. incan will beat LED unless you have low-output.
-awr