Any decent SP should have the talent pain and suffering, talent which makes mind flay auto refresh swp.
As most SP should know that each time swp ticks, the damage is calculated based on current spell power AND any modifier at the time of cast. Therefor if SP gets the stormcloud buff from the shaman first he/she should immediately recast SWP before spreading the buff. If done right you're SWP should have the stormcloud crit bonus even after the cloud wears off you, assuming you constantly refresh it with mind flay.
My swp was crit ticking for 4-6k during the 3 minute fight ;]