The biggest detriment to carrying extra shells on the butt is that it prevents ambidextrous firing. This is not an issue if you chose a sidesaddle style of carrier; you can also load from the sidesaddle without taking the gun from your shoulder. The downside of the sidesaddle is that until you get used to it, the gun feels clumsy to carry at the balance.
The speed feed stock has its own issues. Again, you must drop the gun from your shoulder to access the shells. It is difficult to retrieve the first shell from the speedfeed if both are 3" and for that reason I only load 23/4" shells in mine. The spring pressure tens to break open conventional folded crimps, although it does not seem to affect welded crimps. The springs have no backing, so they are hard on the recoil pad. I fashioned a backing made from a thin piece of stainless steel sheet metal to protect the pad. This caused an unsightly gap between the recoil pad and the stock, but a wrap of electrical tape hides the gap and keeps out moisture.