Quote Originally Posted by Taemek View Post
You are replacing the almost complete cast with a version more powerful, instantly. So essentially, it is as if it finished, just as a Fire III instead of a Fire I. No time is lost and if you are fast enough, you are getting Firestarter off faster then what it would of taken to complete the cast.

Try it, jump and hit Firestarter proc while in the air, it will cast just as it can be cast while running. Soon as you hit the ground, start spamming Fire I again.

I think a lot of people forget or are not aware that interrupting = resets your GCD allowing you cast it instantly no matter where your cast bar is.
But then after casting your Fire 3, you have a global CD of 2.5 seconds. This is where you will lose DPS. Where as, if you let your fire cast, as you already started casting your spell 2.5 seconds earlier: you do not have to wait 2.5 seconds for the next spell.