Are you going to recover from 3,6 mil hit even if it's not an overkill? That's the question. Especially with someone else "sitting" on you and having 3rd NPC shooting 1 mil projectiles. I honestly don't see this as a gear problem. You can argue about DPS perspective (not being able to punch the boss into a certain phase fast enough, etc), but that's basically it.
The difference between dying to a 3,6 mil attack and surviving it is that a healer can top you off if he is quick. It's not the case in solo encounter unless you are blood DK or guardian druid - you don't have means to recuperate your health that fast