I'm mid-ELO and playing Sivir with this build is amazing. Starting off will be kinda hard because of her low health, but if you don't overextend then you'll be fine. Mid-game to end-game, you'll be dishing out amazing damage to everything, and be the goddess of backdooring with your ultimate. I know im going to get bashed by people saying, "you putting your team at a disadvantage by making them fight 4v5"
True, but just autoattacking them wont really do much, and boomerang blade does hurt, but doesn't help your team at all. Ulti helps the team, but they should be able to hold their own without it.
ALSO, IMO, Turrets>Champion Kills
During every teamfight you should be able to at least take out 1-2 turrets. ALWAYS ulti with your minions to maximize damage to turrets. If you die, oh well. ONE person on the other team gets like 300 gold, while your team has taken a turret down, pushed closer to their nexus, AND global 150 gold-300 gold (depending on number of turrets you take out)
Rinse, repeat, and pretty soon the other team will have no turrets and GG.

In the event that there is a teamfight and no more turrets, just ulti, ricochet, auto-attack, and you'll just flat rape. You dont even need good positioning =D.

Mark of Desolation for armor pen
- since sivir is mainly an auto-attack champ, this'll help a lot to take out tanks and rip casters apart.

Seal of Alacrity
- helps for backdooring turrets mid-late game. also compliments ricochet

Greater Glyph of Replenishment
- Lets you use boomerang blade more often. also with meki pendent, ricochet is almost costless

Quint of Desolation
- again, armor pen is beautiful.

standard ranged carry build for offense. defense to compliment her passive

1. Start with either mana crystal, meki pendant, or long sword (your choice) to early build into manamune. I go meki just so I can cast boomerang and ricochet more often.
2. Get tear of goddess ASAP, whenever you get enough gold, just b to base and get it.
3. Get manamune ASAP after tear of goddess
4. Get the Vampric Scepter and start on Bloodthirster (you can get BF sword first if you can affford it, but the scepter's lifesteal will help a LOT)
5. Get Bloodthirster and try to get the +40 dmg ASAP. By now, you should be lv 6 at least and you can take down turrets like nothing with ulti.
6. Start or get Madred's Bloodrazer. (I usually get recurve bow first to help take down turrets. You can get pickaxe first for dmg, its up to you, but remember with recurve bow you'll be hitting more for at least 100 dmg each auto-attack)
7. If games not over, (which it should be if you've been backdooring like youre supposed to)
Get a phantom dancer for the crit, MS, and AS. (Get zeal first, then dagger, then cloak, then phantom dancer)
8. If game STILL isnt over, get an infinity edge and watch everything drop like flies.

You'll need Flash to get away and teleport to help you backdoor during teamfights.

backdoor queen =D