The SPX looks interesting. My Mossberg is an old 'woody' with rifle sights. But my Benelli M1 Super 90 has ghost-ring sights with tritium, very nice. I like the idea of the rail on the SPX. If I were setting up a combat shotgun today, I would want to be able to put an Eotech on it for fast target acquisition with slugs.
IMO you can't go wrong with that setup. Don't know about the porting or accuracy with slugs though. I'd stoke it with 00-buck and put slugs in the stock for special occasions.