Before stumbling on this, I had no idea that when I saved a canned reply, it was saving it for everyone. Otherwise I might not have put my own sig in each of them.
Sorry for the trouble with this. Yes, canned replies are shared with all users who have permissions to use them. It is very useful feedback to find out that it was not obvious that canned replies are shared.
One very interesting thing we are considering is using real posts to back canned replies, that way we can apply group/category security here, elegantly.
The raw markdown of the first post in the topic would be the body of the canned reply. You would edit the post to change the canned reply. It would simply be moving storage from a custom storage area to proper posts.