Im gonna have to do a little more testing..but I feel the nashors is a bit early..

You are a squishy toon with not health reinforcement and sivir eats up mana and you barely have mp5 on there early game.. Also you have tons of AP in the beginning and now her BB scales with dmg I think or something to do with AD...

I think this build is awesome for sivir. I'm doing quite well with her and making great scores. At the begining you must buy a mana regen pendant and the berserker greaves. U must be carefull because sivir at the begining is kinda like paper, very fragile. I also recommend after the berserker greaves, to buy a malady, which you will sell in the end of the game to buy the extech blade.