I've been thinking a lot about Winston (aka Dead) and his Iron Warriors a lot recently, especially since the campaign started. We are both "leaders" of chaos faction, we both play chaos armies that are more oriented to shooting and rely more on Undivided troops as opposed to Cult units and daemons. Yet people live in abject terror of Winston's Iron Warriors and not of my Black Storm.
Some of the reason for that is that I am a good player but not an excellent player. But to be honest, I don't think Winston is a brilliant player either. Yes, he's good and in the top 20 of Deep Space, but I've played him using his Khorne army three time and beat him twice. He's good, but not undefeatable. However, when he's using his Iron Warriors he rarely loses. I however, do not have that problem (benefit?) with my Black Storm Chaos Marines.
So what's the difference? Let's assume that Winston and I are fairly equal in 40K skill (wildly debatable I'm sure but work with me here) and analyze the differences between Iron Warriors and the Black Storm.
1) Extra Heavy Support slot in exchange for Fast Attack slot. Since I rarely use all my fast attack and almost always use all my heavy support, I can see that being a large benefit to the Iron Warriors codex.
2) Ability to take corrupted basilisk and vindicator. Used to be a big deal before Chaos got access to the defiler, but still a significant advantage for no disadvantage. Especially when combined with (1).
3) Can take multiple obliterator squads. Whoa. Really? Wow, that's crazy. Currently obliterators are under priced by about 20 pts per model in my opinion so the ability to take more than three and even to divide them into 2 or 3 squads is huge.
There are other minor advantages and such, but no disadvantages other than restrictions on what cult units and daemons can be used. Basically, the restrictions Iron Warriors face are less severe than the restrictions I place on my own army lists.
Basically my opinion can be summarizes as thus: due to the three abilities listed above Iron Warriors are more powerful than a similarly themed chaos army not using Iron Warrior rules. Are they overpowering? Judging by Winston's win-loss record I think they are very close to it but its hard to say. I'd have to see the army used by other players before I'd go that far.