I'm currently shooting an 870 20 gauge that aippi made up for me.
From having a nearly identical 870P and having fired several hundred rounds through the 870/20, you can configure it in almost any manner you want.
The pic that aippi posted is one such version. I went with the short kiddie stock, a 2 shot extension from Wilson, XS sight, S&J Hardware extended safety and an 8 shot Mesa side saddle. An Elzetta clamp holds a light.
The gun is nimble and very quick to maneuver. The 870P requires thought and movement to swing around and it does rather easily although the 870/20 just seems to show up where my mind expects it to be.
My two beefs, and they have nothing to do with the gun at all, are:
1. A SF integrated forend light isn't available.
2. Mesa Tactical doesn't make a 20 gauge clamp. I've asked Mitch, offered my 870/20 as a testing and development platform however I suppose the market isn't there.
If you want to go mall ninja then a 20 gauge isn't for you but if you want a serious tool that is a capable extension of your mind, then a 20 will serve you very well.