Thunderstorm in PVP and kill order in instancesFollow

Hello, your once and forever newb again. Two questions.

1. Is Thunderstorm effective in PVP? In a Battleground, when surrounded by opponents, would Thunderstorm slow their attacks by 20%? Melee and spell? If yes, is it effective? Or a waste of time?

2. During an instance, the tank gives the kill order, but with the fight started your surrounded by the mob. Even tho theres a kill order, wouldn't Bladestorm be effective? Or should I stick with the kill order and concentrate on the first kill. Bladestorm against 3-4 things would do 30k or more damage in those few seconds. Just sticking with the kill order, when he's done, then you start on a fresh one with no damage done at all.

Thanks for your help.