Lol, Kennen is insanely OP when played correctly, my favorite way to play him though is a DPS hybrid. Your Q will do insane damage :>. Btw I noticed a sivir on enimies team, 1/11/7 is what i think im seeing. HOW? i go 10/2-4/15+ every match with sivir. Go mid, hopefully a mage in mid and just keep on clearing minion waves, blocking mage attacks whenever possible with E as well. Another note for Sivir, Manamune procs W's active, clearing minion waves early game will give you an additional 40-60 attack damage output lategame + will give you insane mana for spamming your Q (with full CDR its very low cd and if played correctly will do 400-800 hp in one chuck (that is if you hit enimies twice with it) its a very strong attack.)
Bloodthirsters are bad if your having a bad game aswell, iv had 3 bloodthirsters on sivir b4. I had TONS of damage, 510 to be exact. Was owning people, but i died and lost stacks on bloodthirsters, sword of occult :"(.
Other key item for Sivir, required at late game is... Frozen Mallet. Bro, its epic. Face it.
Btw most people aren't that great at using sivirs E at a proper time. It doesnt stop on hit affects (like ashes frost shots) but will stop her volley and ult.
Edit:
Her second attack (with passive W active) will proc life steal, frozen mallet affects, madreds, etc. I dont suggust you get any items that have possibility to do 500 direct damage to minions, slows down your manamune farming abilities.