"Better" is a relative term,CC.
All the various 870s with shorter bbls are excellent defensive tools.And,if 4+1 isn't enough to handle a crisis when in trained hands, a 6+1 probably won't either.
A good, close to stock setup for newcomers on a "Serious" 870 would be something like....
If equipped with that egregious set of dimples in the mag tube, these should be removed using the techniques mentioned in the Archives, or for tool challenged folks like myself, by a good smith.
Same same for that keyed safety. The best replacement is the standard old style, the jumbos are easier to knock off and on, IMO. It takes little effort and training to use the standard one well.
It should have a stock that fits the shooter and be equipped with a good pad. The Rempad is OK, as long as it's not calcified into hardness by age.
The trigger should be under 4 lbs for best work, and most are right from the factory.
It should have sights that one can use easily under duress and in any light. The FO jobs are on the fragile side, and most folks can do good work with just a bead. For slug use, a good peep or Ghost Ring setup is better than a bead, but a bead will work if you're up to it.
That's about it. No bellsnwhistles, no complications. The next thing to buy is ammo and range time, maybe lessons.
As for mag extensions, a two shot extension is best for most folks, even if your 870 has a bbl longer than 18". It adds enough weight to aid control, helps stiffen the bbl for slug shooting, and again,4-6 rounds of 12 gauge ammo, properly applied, will handle most anything that a shotgun can, in trained,cool hands.
HTH....