The exact numbers depend on the form of fusion in question, but fusion does have some several places where it has quite substantial x^n growth possibilities, where "n" is definitely greater than one and can be greater than two at times, sometimes even substantially so. This means that there is some real, concrete hope for improvement in a way that, say, solar could never improve more than 4-5x where it is now because the absolutely best it could ever hope for is 100% efficiency. At the core, this is because as you get the plasma hotter and more confined, the rate of fusion goes up very quickly, much much beyond linear increases.

If they replaced the lasers in this building from the 90's with a modern light source it would immediately do two orders of magnitude. Research like this needs to focus on solving, and experimenting with one problem (in this case the physics of inertial confinement fusion). They are not _trying_ to build something which gets "net power out of the building". So don't assume you're net in, net out ratios are representative of what a plant targeted doing that would be.

It's quite easy to see that replacing the lasers, the capacitors, etc. with more modern technology would have an immediate effect. But it doesn't matter until doing the reaction at all makes sense. That's what they are focusing on.