MAXIMUM PHYSICAL DAMAGE (SINGLE TARGET): 46.25 / 83.25 / 120.25 / 159.1 / 194.25 (+ 129.5 / 148 / 166.5 / 185 / 203.5% AD) (+ 92.5% AP) every 3.6 seconds seems a tad too ridiculous.
You are forgetting that lategame is not about spells but about crits for ADCs.
Also not that you are considering max CDR. How often does an ADC have max CDR?
Sivir Boomerang Blade has barely any advantage over auto attacking in teamfights once you finished IE.
Considering how long range other ADCs have I think it's just fair to allow her to make use of her Boomerang Blade to extend her range, even if the damage is inferior to crits.
My main point was that reducing the CD doesn't really fix any of her issues. It just makes her spam her Q non crit ability more often. If ADCs are all about crits then why buff her non crit aspects? Why not buff the aspects of her that generate more gameplay rather than "now you can use your already low CD Q even more often!"
How about more MS on her passive?
How about some new mechanics like Lucian got that one time to her passive, W or Ultimate so she has more flair?
When you just do things by CDs or damage you make every ADC the same. You create a weird arms race.