Stick with factory extension. I have a couple Remington 870s that I put factory extensions on, and I have never had a problem. I tried switching to a Choate extension once, and within a week of the switch, and loading up the mag for about a day, I tried cycling the rounds through the gun. The spring stayed completely compressed and/or got bound up within the tube, and NONE of the rounds fed. I immediately went back to the factory setup. My first 870 is now about 15 years old, and has had the mag extension on it almost from the get-go. Zero problems.
Stick with factory.