the killing speed of a boss is totally irrevelant as arcane.
Your TPS will be the same even if you are the only one doing dps.
For fire, you have a point when you say you reach the 35% mark faster but that's the only moment where you have a tps boost compared a slower kill. In such situations, you have mirror images available to you (remember, it's a speed kill, you won't be able to use it twice).
Just to give you some numbers for the fun of it.
You spend 50% of your time bloodlusted so we have a 80 seconds boss there.
mirror images last 30 seconds.
your tps increase due to a faster kill starts at 35%.
So we can say you did 65% of the fight already (actually, It's gonna be more than that because of the "execute" range but meh, just to make it easier). You just did 52 seconds of the fight and you have 28 remaining. You use mirror image as you get in the tps increase and bam, you don't generate any threat for the rest of the fight, Now, if you do have threat issues before reaching 35%, the problem isn't due to the speed of the kill.
and for the pvp boss, unless manly comes and say he run it in his usual guild run, it's a moot point.