Master to slave volume issues

This has to do with a custom setup where I am using the CCP Mustang drop spindles and rotors. The master cylinder I have is just 14mm in diameter with a 16mm stroke. The rotors are 64mm each x2 or 128mm which makes the volume ration almost 1-10. While this produces adequate leverage and pad force I am limited in the amount of fluid required to move the pistons. I did get around this by adding a 10lb backflow valve which keeps the pads in light contact at all times. While the system works well as is I would like to reduce the ratio to something like 1-6 so the lever would be more firm and the operating range would increase. I am not able to change the master in this application so am wondering if anyone has access or suggestions as to reducing the piston size or changing the calipers to a different style with smaller volume requirements.

Any suggestions appreciated