Personally, I am not really sold on the Ghost Ring sights for IPSC Shotgun. 85-90% of the targets are 25 yards or less, and using shot, in some form or another. Where the Ghost Rings come into there own, are the longer Slug Targets, but usually, a match does not traditionally have a lot of Slug Targets. I find the Ghost Rings are slower to use than just a bead, or traditional Buckhorn sight, for most of the shooting in IPSC Shotgun!
You are going to like the ease of loading the Mossberg though! You will probably need to get the file out and get rid of a lot of the sharp edges, and there are a few other areas to tweak as well to make loading smoother, and racking smoother! Mine is almost as nice as a well used 870 now!