I don't have a ton of experience, but maybe I can get you pointed in a possible direction. As I understand it, two factors may be able to be manipulated to increase the BTUs. One is the pressure of the regulator, and the other is the size of the orifice. Both of those things can affect the force of the "jet" of gas that comes out of the orifice, which affects the way air is drawn into and mixed in the venturi, which affects the BTUs. But, as I said, I don't have much experience with this, so do some more research before you make any changes.
I DO know that orifices are easy to drill out larger, but a bitch to make smaller...
"Oh don't give me none more of that Old Janx Spirit/No, don't you give me none more of that Old Janx Spirit/For my head will fly, my tongue will lie, my eyes will fry and I may die/Won't you pour me one more of that sinful Old Janx Spirit"