T2, I'm sure that ST does fine work, but most of the tweaks and tricks can be owner done or by your local smith...
I'm what Tim Allen would call "Tool Challenged",but I've done plenty of mods and parts swaps on my own 870s and Dept weapons.
Exceptions are trigger work, lengthening the forcing cone, changing a bbl to one that handles tube chokes and installing peep sights. But simple part replacement isn't terribly difficult.
That would include things like adding a mag extension, changing the pitch/drop/length of a stock,swapping stocks/forearms, adding a Sidesaddle,grinding out those #$%^&* dimples in the newer 870s, getting a steel mag follower,etc.
By doing so, you first off learn your weapon better,and,second, save enough cash to buy way more ammo and hit the range more often, which is what most of us need way more than the latest bells and whistles from the custom folks.