Lerner has indeed already considered this …… I already read about this concept some time ago. And you are indeed correct about how it would have a very high specific impulse for acceleration. Which means so little fuel consumption that only a few grams ( milligrams? ) of decaborane could take you just about anywhere you want to go in the solar system and beyond. In fact, I believe the FF is about the only practical way to make an extended mission to Mars feasible.
Not just for propulsion, but the energy required for an extended stay on Mars once you get there. Because it’s a cold wasteland about as cold as Antarctica, even extremely thick insulation in the walls several feet thick won’t hold all the heat inside so the occupants can stay warm. This will require heat supplied by the FF device …… not to mention electricity to power various instruments and other things. An exploratory roving vehicle could also be powered by fusion …… to make hydrogen or to charge it’s batteries. And water could be extracted from martian soil and used to produce oxygen for breathing.
But by far the most exciting concepts is how it could revolutionize the entire transportation sector …… both here on planet Earth and off of it.