SX-1 and Slugs
I have the same gun and choke set up. It works fine with slugs. As advised previously go and try it out to see what brand of slug it shoots best and where it is shooting. The point of impact may surprise you.
I purchased a used barrel for my SX years ago. Cut it off to 18", removed the vented rib. Polished it all up real nice, soldiered Remington Rifle sights on it (as they were all steel), cold blued the whole thing. It looks real good and works like a charm.
In the days when I did all this slug barrels were just starting to appear on the scene, and mounting a scope on a shotgun then was a novelty.
I would not trade my SX-1 for anything, I bought it new in 74 when they first came out. I have put many thousands of rounds through it without a malfunction or any parts replaced as it doesn't need any parts replaced. These were very well designed and built shotguns.