Keith,
As was suggested above, I too suspect that "how" you shoot, your "shooting form" is involved in the objectionable recoil you are experiencing - your gun mount, how the recoil pad makes contact with your shoulder and/or the relationship of your head and cheek to the stock's comb.
I suspect that your stock dimensions may not be allowing you to use a good form, which can easily cause significantly more felt recoil.
The Superposed is not a light weight gun and you are shooting lighter shells so you have done one of the easiest things to reduce felt recoil.
I agree that you should visit a good stock fitter. He would identlfy any flaws in your shooting form and teach you an improved form, then change your stock's dimensions so you could use it. (You would need to remember the form when you get home however, because your gun would be fitted to that form and to an extent, to that form only.
If you cannot visit a GOOD stock fitter (other members would be able to recommend one), consider ordering a copy of my book (URL above).
Rollin
