I think 540 will be a high enough ilv to make attractive for hc raiders to farm tier and trinkets though
So what is t16 normal balanced around? Something makes me doubt that it will be balanced around t15 normal gear.
Guild doing normals maybe 1/2 HC's in tier 15 will continue to do tier 15HC + slowly dipping into SoO.
10+HC guilds now just blowing though the first 2 parts and starting to hit harder content that requires SoO gear only in the end.
Reality: Everyone and his mom will try SoO when it comes out, people will be stuck on the first few bosses, they will complain that raids are too hard for normal, NURFBAT!
According to wowprogress my guilds Item Level: 548.65 (25-man). Clearing SoO in week one shouldn't pose a problem. Even normal mode raiders should be 530 geared going into SoO; with a couple of items from the first 8 bosses should allow them to clear atleast 3/4 of the instance in week one; assuming its only gear holding them back (i mean 530 geared alts really) typically people only clearing 12/12 normal tot either lack time and or skill for HC raiding, so its unlikely players of that standard will clear it in week one, but it should be doable for skilled players on alts atleast.
Flex is going to be staggered release, so I imagine most progression guilds will already have very limited use for it from a gearing standpoint.
That said, I could see progression guilds who get stuck on a boss using Flex as "practice" since it doesn't share the lockout and if a 540 drops that's actually an upgrade, that's just an added benefit.