I have a 12ga 828 U o/u that came with the same stock and "progressive comfort" recoil system.
I was not impressed with the performance of recoil system and took it to my gunsmith and had the the pitch angle changed to my "standard" angle and the stock shortened to give me the correct LOP when I added my usual "Kickeez" magnum recoil pad. I could not hit a thing until I changed the pitch and the Kickeez is, at least to me, noticeably superior to the "Progressive comfort". I do not think you can alter the lop of the existing stock and keep the "Progressive comfort" but they/Benelli have one choice to shorten and that involves a new expensive butt stock to maintain the "progressive comfort" system. I am very happy with the results and had the same thing done right away to my 2nd 828 U.