Another thing to keep in mind about fuel economy is that fuel is effectively, if not literally, free. It's about as expensive as "solar energy" fuel... just gather more!
Hydrogen is the most common element in the universe, and the time required to refine it into transit drive-grade fuel is the real "cost" to the fuel. One of the rule books (I believe SO) even has rules for how long it takes for a fusion drive to just crack water down into DropShip fuel (doesn't even have to be potable water, so it's still viable even on "water poor" worlds)
Worrying about military vs civilian drive efficiency is probably going to be akin to worrying about 34 vs 24 MPG when gas costs under a penny per gallon... not a whole lot of money to be saved by spending a lot of money on improved fuel efficiency...
The real advantage in practical terms of a military grade drive is endurance for the same amount of tonnage devoted to fuel. But civilian DropShips are oozing spare tonnage.. if it's truly necessary they can always pack a couple hundred, or thousand, tons of extra fuel in cargo...