Given that some 535 geared players have the performance of a 490 geared player, I'd say yes, it's definitely reasonable to demand high ilvl.
If you can find enough people it's fine. If you're lower geared, just play with people in your ilvl range and befriend pros.
"There is a pervasive myth that making content hard will induce players to rise to the occasion. We find the opposite. " -- Ghostcrawler
"The bit about hardcore players not always caring about the long term interests of the game is spot on." -- Ghostcrawler
"Do you want a game with no casuals so about 500 players?"
i did a flex run, 2 people had legendary cloaks, i have a 526 ilvl and i smoked them in dps. but i'm the skilled long time player who doesn't invest enough time into getting normal raid gear.
550 is probably excessive, but I don't think 530 or 540 is really out of the question. I've got 542 ilevel and could still use a few items from flex. I could also rock out some offspec gear.