Where'd ya think we got 'em, Dave?
{'sides, if we repeat the mantra enough, can we be in your book?} 
Assuming shotgun fits right, I can't think of a single add-on that makes it shoot better - aside from perhaps a decent trigger.
Many are nice & some, even real helpful for SD (flashlight mount might be one), but a good stock pump's about as good as it gets.
The rest is always up to the shootist.
I've one 870 set up for SD & the only thing extra is an extension tube. The rest are factory stock. & it really wouldn't matter which one got grabbed ....