Jinx is one of my favourte ADCs (and i don't play that role so much, but i really enjoy playing Jinx) BUT this buff is just bad!
I'm talking about the buff where W can now critically strike.
1. RNG is bad in general, and that's a point even pro players are pointing out since season 4( i started playing end of season 3 and the "remove crit pls" drama was there AT LEAST since then)
2. It's not fun for enemy players neither for Jinx player: You mostly have 50% crit strike ( hurricane + IE, maybe sometimes you go for another crit item in late game instead of a defensive item, but for the most you have 50% crit chance), that means that for the entire game, you have 50% to do a massive ammount of damage with the zap ( the zap scales pretty good, and IE make crits stronger than just 200%) and 50% of doing the standard damage.
So: I'm low healt but i know that a standard zap is not gonna kill me, but a critting one instead will, i have to flash it or not? Probably yes! But feels bad because mabe is a wasted flash. Instead, i zap someone and i know he can't flash or dodge it, i crit and kill him?! COOL! I Don't crit and he surivevs with 100HP?!....i tilt xD (especially with a 80% crit chance)
This is a RNG buff, is unreliable and i really don't like those type of buffs,also the late game critting Zap will be pretty busted as a damage, and i really don't think Jinx problems are on the damage side xD (sicnerelly, i think Jinx is in a good state right now, maybe th meta in the bot lane is not good for her but she is not weak IMO)
Furthermore, Zap is still good to snipe down targets pretty far away( without having to ult every time to kill someone that flashed away with low healt), it scales pretty good, and slows too, i don't feel it's a lucklaster spell at the moment. That doesen't mean is a spell that needs no changes, i stil think is the only spell that doesn't fit the playstyle ant the personality that much (too slow and "sniping" to be in the Jinx playstyle and personality, perfect for Jhin( he has a similar W) because he is a methodic assassin and a sniper, but Jinx just runs everywhere and blow stuff up).
IMO (if you really think Jinx needs some buffs and the W some changes) you may try to give Jinx a little (decaying) burst of movement speed if she hits(or just after using the zap) someone with the W because sometimes, even if it slows, having to stop moving for a second when kiting feels pretty bad. (making the animation way faster will be the best IMO, but will remove some time to react for enemies so you should have to slow donw the projectile speed to compensate :\ )