I'm not sure TJ has got this right.
As I understand it, using the given example:
A 30W halogen bulb with a 12V battery would indeed draw 2.5A, however to run an LED like the MCE you need a driver such as the Maxflex. This works in a completely different way.
Power consumed by an single MCE LED is 3.5V*0.7A=2.54W
(3.5V is the forward voltage when drawing the max forward current of 700mA, not 1A as stated)
If you assume an average efficiency of 88%, input power is 2.54/0.88=2.88W
At 12V that gives a supply current of 0.24A.
Therefore the run time will be approx 10.5 times that of a halogen not double!
I can't comment on the specifics of halogen vs LED brightness comparisons but I would expect a quad XP-G to be WAY brighter than 50W halogen.
Incidently, the quad XP-G at full tilt (1A) would be WAY WAY brighter than the single MCE, but as TJ says wouldn't last as long, in fact if I've worked it out right only a 5th of the time.