Gday
I have been working on exactly the same problem ( but only 1A ), for powering a small surveillance video camera from a battery ( attached to a telecope )
My input range is 14V down to 10 as the battery flattens and i need a constant 12V +/- 0.1V.
I have indeed gone down the DC-DC stepup, then linear stepdown ( LDO ) route, as space is at a premium, but if you have room, a DC-DC flyback system may be better.
Another advantage of the stepup then step down is the LDO stepdown unit will act as an output filter.
With my current setup, i can average around 76% efficiency across the bulk of the range. Your main problem will be when yr voltage exceeds say 12.3V, as the stepup unit stops switching, and all heat loss comes via the 12V stepdown. The higher yr input voltage, the better a heatsink you may require on that unit.
Andrew