The HIGHER the unfired cylinder pressure, the higher the power.
The LOWER the unfired cylinder temperature, the higher the power.
Higher unfired cylinder pressure requires LESS timing (the opposite of why you want vacuum advance at a cruise) and that means less negative work. Until you get to the point where you are using more work to compress the unfired fuel/air than you gain on the work side.
The extra help from a greater expansion ratio is a nice bonus.