High ilvl requirements are because legendary cloak buffs ilvl, so anyone under 540 is usually useless.
For getting back into raiding it's really the same as it's always been, if you're legitimately a good player you shouldn't have any problem. I was only 550 ilvl a few weeks ago and because I had good logs from earlier in the xpac and past xpac experience I got into a guild that's 11/14 heroic and I'm close to being caught up in gear.