The end goal isn't to preheat the fuel, it's to keep the heat from escaping, because you want all the heat to go into the sodium.

The heat is being used to generate electricity.

Fuel is burned to head sodium, if you are getting too much heat for your taste you can burn less fuel. It is kinda the goal of the exercise.

But in any case, I believe that the more you heat sodium, the more light it emits, probably there is a practical limit on an incoming heat power after which the thing will go boom, but before that it will follow some roughly linear law: the more heat energy in, the more light comes out. Though I'm not a physicist, so I make be wrong, even if I do not see how I can be wrong.