As i've written above, SI is signficantly worse for late StM for various reasons. SI is target capped at one late into StM, while AS single-target is already 3-4 times more Insanity than what SI provides, scaling with targets.
If you want to get the most out of StM, you have to pick AS (this just doesn't mean it will be the highest Encounter DPS, just the highest StM DPS)