Well, the general-ish rule of thumb would usually point to lighter pellets being more efficient in a springer and heavier ones in a PCP. But there will always be exceptions. And I used to use the 9.3 grain Silhouettes in my springers years ago. Really good. And excellent in the wind, too.
A good testing session with those Heavies will be most enlightening, methinks, and they really were very special in terms of beating the Hollow Hoolies.