also you lose a ton of insanity during stm. When your dots start ticking in the 1/sec range and you have about 20%+ crit or so, the gains will be massive. (think about the gains, children!)
coming back to other content:
I don't see san'layn in it's current form beat out as in any form of content other than world quests and dungeons, where there isn't enough time spend in void form anyways and travel times make as undesirable. But then you may also think about using shadowy insight...
But more sims will come and preach to us the right way of the spriest.