Sure, in principle, but in practice you are limited by the actual fuel you need to burn, and it seems the existing designs get close enough to those limits and

Sure, in principle, but in practice you are limited by the actual fuel you need to burn, and it seems the existing designs get close enough to those limits and do not stand to gain significantly from higher energy densities. Fuel efficiency and range is a primary target for optimization for decades, I doubt there are vast reserves of untapped efficiency just waiting for a hotter engine.