Before I rant --- I love the idea of flex - I think it should be implemented for all difficulty levels.
I hate having multiple difficulty levels. Does anybody seriously enjoy killing the same damn boss 100+ times per tier?
Solution: One difficulty (heroic) with flex. Every month, any bosses that have been killed by at least 100 different guilds take a 10% nerf. Top end guilds can still push for rankings without worry, and the casuals can still see all content before next tier, and EVERYBODY gets excited when they kill a boss (if anybody even remembers that feeling)....
And who cares about perfectly balancing flex --- let the top end guilds play around with the numbers & compositions themselves.