"Suppose I wanted to use antimatter for propulsion. The reaction wastes something like 40% of its energy producing neutral pions which almost immediately decay into useless gamma rays. What if the protons & antiprotons were first accelerated to relativistic speed, wouldn't the gamma rays produced be redshifted to a more reasonable wavelength, say infrared?"
No clue. :)
But "all you need to do" is accelerate particles to near light speed to get the "perfect" rocket. Accelerating lots of particles "closer" to light speed is "why" you need a lot of energy and why you use anti-matter.
The other factor is the size (mass) of the device needed to accelerate these particles, it's mass (as part of the entire vehicle mass) in relation to the amount and of the speed particles exhausted determine how efficent it is.
Anyways, why are the pions or gamma rays useless? And why would infrared be any better? And why wouldn't these particles be already accelerated near light speeds, aren't the relativistic speeds causing small delay in decay of the pions?