5.4 ilevels aren't finalized but we're thinking something like 528 LFR, 536 Flex, 553 Normal, 566 Heroic.- Greg Street (@Ghostcrawler) June 12, 2013
Do note, first and foremost, that these numbers are not finalized. They should be taken with a very large pinch of salt, and are subject to considerable change. The PTR isn't even live at the time of writing this post, so let's not get ahead of ourselves and proclaim the end of days for raiding.
So let's look at the current item levels. Raid finder drops 502, normal mode 522, normal thunderforged 528, heroic mode, 535, heroic thunderforged 541. So, the gap between normal mode and LFR has been widened, from 20 to 25, with Flex sitting 17 levels lower than normal mode. The gaps above that are the same, but there's been no mention of something like Thunderforged gear.
For heroic geared raiders, Flex seems like a logical difficulty at these levels, but there will be some theorycrafting that needs to be done to ascertain whether the new tier's potentially lower-level drops will outdo their current gear thanks to set bonuses, trinkets and the like. What this ilvl distribution seems to be saying is that heroic-geared raiders shouldn't need to run LFR at all. But, this is all subject to change.