Burley,
If you have some tin or silver to add to get the hardness up slightly it will improve the shape of the cavitation because both will increase ductility of the alloy.
With the alloy you have, maybe go another 1-20 and see if you can get it to 12BHN; you may get nice furled back mushrooms.
Adding mono will definitely get it to 15, but the crystalline nature of antimony will make the cavitation by powdering down and less weight retention, not a classic mushroom. This is especially true if antimony percentage is greater than tin and the impact speed.
Either way you are going to have expansion and greater cavitation, it just depends on what you are happy with.
