I have had Briley Thinwall chokes put in three of my guns (one 12ga 870 Wingmaster Mag), my fathers 1100 which he still uses, and my 1100 LT-20.
All of them work great. Of course, I didn't do it myself.... use the gunsmith
But I have been very pleased.
Value wise, it does demish the "worth" of a gun... but since the gun you have is only worth a lot to you (no offense, but they made a lot of 870s), then its definately more than worth it to make it more useable. You will get so much more enjoyment out of it if you get to hunt with it more often.
Good luck.