I am using PIE and SS, PIE because well, need moar MPs! Spell speed because almost every single ability we have is on the GCD, while you need a large amount to notice the different, when stacked high enough that extra few split seconds is the difference between a dead tank and clutch cure. Moral of the story: More spell speed makes healing more forgiving for a WHM.

P.S. It also buffs PoM.