A 50W halogen lamp, properly driven, should put out something on the order of 1000-1500 lumen, depending upon things like lamp gas fill, operating temperature, etc. The new 5W lamps are expected to put out about 120 lumen each, so the actual efficiency of the LEDs will be about the same as the efficiency of the halogen.
You would get a whiter light, which would be easily and efficiently dimmed (you can dim halogen, but you lose big time on both color and efficiency.) Lamp life would also be much longer...of course you are talking quite a few $$ in order to get the illuminating power of a single $10 halogen.
If you want to get the best bang for your battery power, I'd look at the HID dive lamps. The 10W HID systems won't provide sufficient light, but the 20W lamps put out about as much light as a 50W halogen. You pay quite a bit for the bulb, and the bulb life is nothing like that of the LEDs, but over time the batteries are much more expensive than the bulb (even with rechargeable batteries; they wear out just like anything else.)
If you decide to go the LED route, I'd strongly suggest looking into current regulating switching power supplies rather than voltage regulating supplies. This lets you set LED current without having to worry that the LED forward voltage will change with time or temperature (or pressure, for that matter). Alas I cannot help you with boost converter circuits at these power levels.
Good luck!
-Jon