I think I was underestimating how much energy could be lost to x-rays on some of those. D2-He3, the aneutronic fusion everyone likes to talk about is still over 80% efficient but H1-B looks really bad if you're not driving a heat engine with it.

Yes, though the H1-B reaction itself emits no electromagnetic radiation at all. The x-rays are from bremsstrahlung radiation. Some boron fusion projects think they have ways to avoid that (which is necessary to avoid the plasma cooling too fast to achieve net power).