A few thoughts:
Consistency between shots is ok. The size of the reballs make it hard. Have you cosidered 677 or 675 freak inserts? Also ty mcneer here on mcb makes brass inserts that are 670 or smaller. Either of those options will improve the chrono numbers.
Why not respring to get a velocity of 300 - 310? Then you can use the stab to reduce pressure and velocity to feild legal speeds. My understanding of sterling is they are most effecient at an input pressure of around 550 psi. That should ensure lots of flexiablity ( indoor /outdoor). My 1/50th of a dollar.