-9 was a really good score for me. That's when I was coaching and didn't have the shoulder issue, so I was practicing it at least once a day or more (I had a hoop at home and work). Practicing it helps because you are really laser focused on swishes, not just makes. I remember one of our players saying, "wait, you can try and swish it?" It makes you automatically begin to shoot with more arc and look for that "perfect motion". As a result, a ton more of your "misses" still go in. I'd guess when I was practicing almost daily, I went from a 70s % FT shooter to closer to 85-90%. And, I never thought of myself as a shooter.
Oddly, with my shoulder, throwing is still pretty good unless I'm really trying to thow hard, which I just don't do anymore. Shooting doesn't cause me pain, it just has a "catch" and a bit of weakness that makes my a much worse FT shooter.