As a "fuel", yes, it is pretty efficient. That said, the "optimistic" version of the spaceship (representing the upper bounds of that specific technology), still came in at 400,000 tons, after a sort of miniaturisation. That's a lot of spaceship, and as far as I know, none of those calculations cater for launching from the planet's surface. If nothing else, that would require additional fuel, and it's not negligable, because each ton of extra fuel is pushing the 400,000 tons already loaded. It's a game of (greatly) diminishing returns.