Usually Blizz nerfs the final raid tier before the xpac is released. With both DS and ICC they had the stacking debuff and by the time 30% hit it was quite sizable. While we have had the item upgrades, they're not quite the same as it doesn't benefit newer players as much or alts.
I'm thinking maybe one month of a 10-15% nerf and then 1 month of 6.0 seems reasonable.