I have a mag extension, took it off. It completely changed the guns balance when loaded. I'd rather have 4+1 with a gun that still points and shoots where I want. I don't even think that is an option on a 500 anyway
You're correct that extensions aren't an option on the 500, you'd have to get an entirely different tube and barrel if you wanted to change the magazine capacity. So I'd agree with others that the best items on a defensive shotgun are a side saddle and light. As far as a sling goes, I personally do not like slings on my shotguns because there are too many ways for it to interfere with weapon manipulation. Another upgrade I think is worthwhile on a defensive shotgun is a better set of sights. I personally like ghost rings because they give me the option to use slugs and extend the usable range of my shotgun out exponentially. But that's really only a necessary thing if you plan on using your gun with slugs as well.
I will say the most important thing you can do with a defensive shotgun is spend the money to try various loads and actually pattern the gun. It takes some time and money to shoot your way through various loads, but it's well worth the time to find what will actually shoot and pattern best from a gun you plan to use for home defense. I was surprised to find that my Mossberg 500 patterned best with some really cheap 00 Buck, where most of the other guns I've shot have done better with more expensive ammunition. So take the time to pattern it and see what YOUR gun likes, it's well worth the time.