You'd muck up the smooth handling qualities of the 870 for a gain of only a few rounds, and questionable reliability? Most civilian long gun fights last all of one round, and at short range. I'd rather have a slick gun that is fast to bring into action than some clunky thing with a huge box under it.
My own 870 is box-stock - no extended mag, no ghost ring sights, no clunky Sidesaddle, no million-lumen headlight. I've tried all these things, but find them counterproductive to the shotgun's real mission in civilian defense.
Which is not what you asked, of course.