Perhaps with our limited 21st Century understanding of the physics involved. The Romulans may have had thousands of years to get it to work well enough to be a good choice for their needs. No doubt, their fusion reactors have the same historical roots as the Vulcans which no doubt are at least conceptually represented in the fusion reactors in modern Federation impulse drives.
For my money, "Real Science" has only a back-seat informative role when massaging the numbers for the magic we see in shows like Star Trek.
I've been trying to let this notion roll around in me gob before replying so as I don't spit out a knee-jerk reaction.
But my first thought remains foremost in my head:
While this may be a valid argument for many other parts of Star Trek.... Nuclear fusion energy comes from the difference in mass between the fuel and the exhaust. If the energy is coming from somewhere else, it's not nuclear fusion. In order to get a mass to energy conversion of 1% or more from a nuclear fusion reaction, the mass of the particles would have to be radically different than what we know them to be and, therefore, the laws of physics would have to be radically different. In which case, we're arguing angles on the head of a needle.
Now, we could speculate that "fusion" does not mean "nuclear fusion" in which case we've got a completely different discussion on our hands. But as long as we stipulate "nuclear fusion" we are stuck with less than a 1% mass to energy conversion. And, thus, stuck with tankages 100 to 1000 times less than M/AM fuel.
Unless, as I speculated earlier, we stipulate ram scoops.