Hello ! I'm a 300k mastery point Xayah player. I know that doesn't mean much, but I've found that I tend to do the highest damage in the game with this build. The point of this build is to get out as much damage as possible in a short amount of time. I've found that Krakenslayer works really well on her because it buffs up Deadly Plumage. It also helps shred through tanks, and deal with squishies. Navori is good for reducing Xayah's cooldowns, which allows her to put out more feathers. You'll be able to use Deadly Plumage every few seconds in team fights. I've gotten to have 19 feathers out at the same time with this build, which does serious damage against anyone you face when you use Bladecaller. The Collector is nice to have in case you use Featherstorm and aren't able to root all of your targets with Bladecaller. The Collector should be able to finish them off once you use those two abilities.
Bloodthirster is always nice to have on Xayah in case you don't have a healer on your team so you can sustain during fights. The longer a fight goes on, the more Xayah benefits from it. Infinity Edge, Mortal Reminder, or Lord Dom's can give the last bit of Crit that she needs to reach 100% Crit. Mercurial's also gives Crit this season, and can help Xayah get out of sticky situations if she doesn't have her Featherstorm to escape CC.
I'm going to get eaten alive for this, but if you have a support like Lulu that can buff your attack speed, or someone who builds Ardent, another option is to ditch the Berserker's Grieves and get Swifties to allow Xayah to reposition to line her feathers up faster. Your attack speed will be, at minimum, 1.51 (without Lethal Tempo, Grieves, or Deadly Plumage active) at level 18 if you're full build. If you use Deadly Plumage and your Lethal Tempo is active, this moves your attack speed to 2.54. This is also if you don't get Mortal Reminder, which gives a 25% attack speed buff. With Mortal Reminder, your attack speed will be 1.66 before Deadly Plumage and Lethal Tempo, and with, it increases your attack speed to 2.70. This is all without the use of an ardent support or Polymorph from a Lulu. However, I still, for the most part, recommend the purchase of Grieves.
Also, the speed at which Xayah can output her Double Daggers scales with attack speed. The more attack speed you have, the faster you can go back to auto attacking. A tip to cancel the Double Daggers animation early game to get back to attacking is to press your Deadly Plumage at the same time as your Double Daggers. This not only cancels the animation for Double Daggers, but it also facilitates in getting 3 feathers out as soon as possible to get a root in. If you have feathers left for your passive, Clean Cuts, you can auto and flash at the same time to get a feather out for a root.
Xayah's biggest weakness with this build will be assassins. However, if the assassin isn't able to burst her down, she will be able to eliminate them from the game. With a good front line, the game should be won once she gets Navori and Krakenslayer.