Really depends on your healers, and how well your people follow directions in general.
Naj'entus is a major gear check, and there are some things to take into consideration:
1. Can everyone in your raid get 10,000 HP? Najentus does an AoE for 8,500 and can target someone randomly for about 2k right after that, so unless you have 10k health (or damn close), you're going to die.
2. How well do your people take orders and pay attention? This isn't just a pew pew WE WIN fight. He'll do an attack that "spines" a person, doing initial damage and aditional DoT damage until someone removes the spine off of the person. That spine then has to be used to pop Najentus' bubble, which triggers the 8.5k damage. Oh, and if you don't heal everyone up to above 8,500 health QUICKLY, you'll lose, because he heals himself while bubbled.
3. How are your healers? It's really a very healing-intensive fight. Think of how hard Anetheron is on your healers, and make it even harder.
I don't remember how long my guild waited until we went from Hyjal to BT, but the difference in difficulty is noticeable. Unless you have Anetheron on farm- no problemo, you laugh at him - then you might want to hold off. Kaz'rogal is a much easier fight.