Your winrate with Sona is actually the best among your "main" ranked champs, so you're doing something right there. I do agree that buying lots of AP is probably wrong. But, in B5, who is actually making use of the vision you provide? The advice of others is good, buy more wards, less items. I expect that actually volunteering to play support gives you an advantage over the "support" Nidalees and such.
One of the bigger problems I see if your average death rate. It's 5.9 on Sona and over 6 on every other ranked champ you play (except the one Ahri game). That's too many deaths per game, consistently. You need to learn to play much more safely, in general. On non-supports, you're picking up kills, but handing the advantage right back. Not dying is probably the best thing you can do to improve, that I see.