Aka, everything under challenger,
I'm sticking with my "up to challenger" assessment. I'm pretty confident in that one. She's really doing quite well in diamond right now.
Diamond and above is usually where things get really hairy and people have the quick reflexes and extensive game knowledge to punish Sona for living, not just Challenger. Out of curiosity, where are you gathering your statistics? I'd seriously like to see them.
In practical terms, yes it has.
5 movement speed sounds like a little, but it can make a big difference — especially on someone like Sona, who can't really get around that easily. I felt that downgrade hard when it came.
She's also more vulnerable to getting killed early on now with the lower HP, and supports aren't known to bounce back that well. That'll probably still be the case even after the EXP changes coming, and definitely the case on one as gold-reliant as Sona is.
A lot of people confound terrible design with a poor balance state.
This is true. Sona, Volibear, and Skarner all have evident design flaws (Skarner in particular had an entire thread from Riot discussing it not too long ago, after his most recent update).
Volibear and Skarner are bad, just not in terms of strength.
Sona is bad in design, and as a result Riot's tried their damndest to make her bad in strength as well (unlike with other champions). Every time up until recently when they've talked about her, it's reaffirmed that they don't want her to be strong, because she'll automatically win if she is — and because of her design, that's pretty accurate.
This is where I'm going to have to cite "everyone disagrees with you". The main reason Sona gets picked is dominating a lane. Her scaling is really godawful, and that has been a major aspect of complaints about her.
She definitely gets picked to dominate a lane, but dominating a lane gives you an edge that you can and should leverage. If she gets gold, she scales with it hard, because (as part of her bad design) she's a support that needs it. If she can't win lane, she suffers harder than almost any other support I can think of — especially champions designed specifically to be a support. If she can, she rolls with it pretty heavily.
I mean, who wants the E to be a one-hit range increase? I think it's fair to say that this change is coming out of left field.
That was something I hadn't actually seen before about thirty minutes ago and, honestly, the range increase is something she desperately needed to make use of Power Chord without immediately getting chunked down to 1/3 HP for it against anyone competent. If her E is still a speed boost especially (it'd be weird if she didn't get the ally-boosting one), that gives her very good hit-and-run capability with E > Q > Staccato Power Chord AA.
I really don't know about putting it on Song of Celerity specifically, though. I feel like it might be better in terms of balancing to give each variety of Power Chord its own range increase scaling.