Isn't the pitch the same thing as the drop. When talking pitch you typically are referring to angles? The drops are measurements corresponding to those angles. The drop at Comb is the distance from sight line down to stock where your cheek rests. The heel drop is the distance of drop from sight line to the top of stock at the butt plate.
If you change the angle (pitch - shims)) you change the two distances.
Most americans perfer comb drop of 1.5 inches or so. The original SBE's has much less than that because Europeans don't like drop. They like a flat stock. This causes Americans to have to scrunch their head lower onto the stock to look down the barrel. If they are not looking down the barrel they shoot high.
I know the angle could be cut, but does that change the forces from the recoil and the location of the recoil, does the top part of butt plate get significantly more forces than the bottom and will this damage the stock over time?
Somebody experienced in "stock-smithing" and benellis will have to answer the question of how many shims, can you cut the stock and not damage it, etc.