I'm still in the process of filling the roster for our guild. We just started up a couple weeks ago, and we clear Normal, but we're still 4 people short that I have to pug each week. Occasionally I get lucky and someone from my friend's list joins who I know, but much of the time we're stuck using OQ or Openraid.
That said, in my experience, 95% of the WoW population's DPS does dps befitting someone 20 ilvl below what it should be. Meaning, someone with 550 ilvl, I assume they're going to DPS like I did when I was 525-530, and most of the time this assumption holds true. Therefore, when I pug for our Normal runs, and especially Garrosh, if you're under 560 I probably won't choose you unless you make a good case as to why I can trust you to play your class properly.
Hell, I can't even get decent DPS out of 565+ pugs. When I get 565+ shamans, warlocks and mages who can't even touch 200k on Garrosh AoE you really can't expect me to just pick up every random person I see with a 550 ilvl.
Can Garrosh be done with 550 DPS? Absolutely. It can be done with much lower ilvl DPS than that. The problem isn't want CAN be done, rather it's with what players can do. And most players can't play properly to match their ilvl.
So yeah, if I'm using OQ and you're 550, there's no way I'm inviting you. You're a stranger, and 95% of the strangers I've ever played with can't play for shit, and I can't gamble my raid group's time inviting every mediocre ilvl character on my OQ list to "try them out". With that said, I also look in Openraid at the same time, and I'm occasionally asked by lower ilvl toons to join, and if they can give me any indication that they can play properly, they get the spot.
Now, tanks and healers are a different story. It doesn't take as much ilvl to compensate for tanks and heals in Normal as it does for DPS. So I've taken tanks as low as 528 (and cleared most just fine) and usually healers in the 540's. (Hell, I was 540 when I started healing Normal).
So yeah, the issue (for me at least) is not whether or not it can be cleared in 550 or below (half the people I got were in the 540's when we started raiding together and clearing just fine), but rather the complete and utter lack of knowledge that much of the playerbase has on how to play effectively on a level befitting their ilvl.