I have a launch 6S as well, and did the free battery replacement under the recall program. It still performs great aside from quirks of iOS11 itself.
If I were you, I would wait for 11.3, and then see the battery performance. If it needs to be replaced, do so, and you can probably enjoy the 6S for another year or so.
The 6S is a great phone.
It is the first iPhone with 2GB of RAM.
It is the first iPhone to support ARKit.
I can see it still performing well for another year or two, and even then I can see it performing well even on its last supported iOS version (kinda like the iPhone 5).