Remember that max pressure in a shotgun comes well inside the chamber. By the time the shot charge has moved far enough to enter the forcing cone it has already begun to fall. There is little reason to believe a short cone will cause an increase in the max peak pressure of the load. It "May" have a very slight effect on slowing the fall, but that's about it.

No, I am not a Ballistician, so anyone who has "Proof" this is wrong please do so. So many times I am reminded of the words of Julian S Hatcher when he said There are things which Sound good when you Say Them Real Fast, that simply won't hold up to Scrutiny.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra