I agree with the the OP. I find she is still viable, and I can make her work pretty well. However, I have to work very hard to get any gains out of her compared to say ... any other carry. I think she needs to have some cdr and mana costs reduced.
The damage scaling was reduced big time on this, so it's time they removed the exponential decay. If they don't remove the exponential decay, then they should return the scaling to what it once was. I can watch my Boomerang go through the creeps and hit the champion and do essentially 0 dmg.
On top of the dmg issues, it is slow and the cost is very high as was mentioned. All things considered, it is probably the hardest spell in the game to harass with now. You have to be insanely good with it to get any rewards at all. I wouldn't have a problem with this, except champions like Karthus exist who can spam their q over and over and over with 0 skill until eventually they catch you with one.
Second: Spell Shield:
It has way too high of a cdr. I love this spell, but it only blocks one spell, and I am pretty sure every other spell in the game has a lower cdr by at least half until it reaches level 5. Thus, you shield the first spell (with great timing and skill I might add), and then they just spam it three more times while your shield is coming back up and guess what? You still get nuked to hell. By level 3 it has an almost reasonable cdr, but really they just need to adjust the cdr over all on this ability.
Garbage passive. Arguably the worst in the game. Unnoticeable, and utterly useless. Just get her a new passive already.
Has some power, but needs to be looked at again, it is definitely out classed now.
Her value is in the fact that she can farm much better than most carries, but good luck having a situation where you can stand idle and use your ricochet to benefit from this (the higher in ranked you go, the less likely you can stand still even for a second). Sivir does still hold a niche, but with the invent of Noc with a spell shield and a 3 second cc, she is looking really really out classed. Please look at this champion and make her useful again so people don't laugh at me for using her -_-.
Note: They don't even queue dodge, they just laugh and say 'gg, I gotta see how noob this Sivir is'.