Skyactive using retard intake valve close to reduce pumping loss. This also reduce Absolute CR. You can reduce Absolute CR by just retard it more.
The more you retard intake valve close, the more you increase expansion ration. Hence, improve fuel efficiency. However, doing so need to reflash your ECU firmware with new VVT table. And Mazda ECUs are encrypted. Even in Mazda 2, no one can break its code yet.
Even you could do all that. The more you pushing the limit by advance ignition timing and retard intake valve close, It could lead to destructive of your entire intake system by backfire. And inadequate retard valve close would lead to detonation inside the combustion chamber.
Is it worth the risk? I don't know. It's depend on your point of view.