You could buy a new barrel??? Heck, even Mossberg makes 870 barrels. I dissagree, sight radius is irrelevant on a shotgun. Why? A; you shouldn't be admiring your wonderful long shiney serrated rib in the first place. B; there really isn't a rear sight to open or close "air" between front and rear sights (of which there are none on most shotguns).
There was a fellow who had just come from a gunshow with his first shotgun, a Beretta FP1201 with the 20" barrel and rifle sights. He shot suprisingly well with it, so he let me give it a go. I found that rifle sights, oddly enough, work just as well for skeet as a plain jane rib. This may be because I might not have been focused on the sight, rather the clay.