The absolute value of haste doesn't decrease. However, it shall be said that the value of other stats (ie spellpower and crit) grows as haste increases. That's the case for most stats (they increases the value of other stats), and that's the point of balancing stats.
I just have to quote this, as its the new ShadowPriest reality. We're not playing 2.x (TBC) nor pre-3.x ShadowPriests anymore. We actually scale decently with more than just 1 stat. The only really cappable stat (given values within existing ilvl gear), is Hit Rating. We are no longer stacking just Spell Power like in TBC.
When it comes to the part about Haste Plateus, they are virtually gone, thanks to hasted Devouring Plague and Vampiric Touch. The Plateus were never that huge anyway. Reason for these Plateus were mostly spell collisions.
Just make sure you cap that Hit.