A ramjet is the fastest engine that slows down the inlet air into a combustor. To go faster requires inline combustion without slowing down the airflow, aka scramjet. Another idea is an external combustion engine with a starshape, but I've never seen one built, not even a mockup. The linear aerospike engine bears a crude resemblance to an external combustion engine, but of course it is a bipropellant rocket engine, not airbreathing.

Ultimately I don't think it is worthwhile to even do the calculations because the stagnation temperature at the leading edge would be so hot as to disintegrate nearly any material we've got. Ceramic ablative coatings might stand up for a bit, but this is the inherent problem with hypersonic craft, rockets, etc, the operating temperatures and stress cycling make the engines inherently non-reuseable. They just don't last long enough. And they are expensive to start out.

Rockets, and airbreathers have their role in the future of aerospace, exactly what they are doing now. But if we want to do more we will need to develop radically new ideas, of which rockets are merely a component, specialized and miniaturized.