Replied to you over on the Benelli USA website.
The D shim offers the lowest drop, A the least.
So maybe try a C then B to get what you need.
Cast is harder with the SBE, because there is no factory shim kit like the SBE II has. But, with an extra shim, and some sandpaper or Dremel drum sander, you can grind away at a shim and get a little cast.
You can also mess with the front bead, larger will offer more drop, smaller will offer less drop.
Or, you can even put a sight on it, that's what many of my friends do with a gun where POA doesn't match POI very well...
Good luck!
mudhen