Tom, I too was confused by rehunn's post. Superheat improves the thermal efficiency of ANY vapor cycle. Your comments regarding superheat increasing volume are true...but that isn't the reason it is more efficient. Let's look at a sample thermodynamic cycle for an engine with superheat:
Notice that all heat addition is essentially a constant-pressure process. Heat addition increases the energy in the fluid which is manifested through an increase in temperature and entropy (s) once you are in the vapor state. The perfect gas law indicates V=nRT/P so when all else is constant, an increase in T results in an increase in V, validating your comment. However the real increase in power and efficiency comes from the basic fact that we have extracted more energy from the "fire" and have more available energy in the fluid to do work.