Even though it's not worded in the Mastery specifically in the patch notes, I would have to think that Insanity would also benefit from mastery, as it does a percentage difference of Mind Flay damage that IS affected by the mastery. So double damage, mastery buffed mindflay ticks (aka Insanity), will be more than Mind Spike whenever they get around to balancing numbers.
As long as they have Mind Spike be just under Mind Blast, and Mind Flay just under Mind Spike, then when Insanity is applied, it will be in the rotation. Especially with DP ticking during the Insanity it will be greater than casting mind spike over the same duration. Just have to have Mind Flay close to Mind Spike, and with DP and Insanity it'll push over Mind Spike DPET
90 Talent > Mind Blast > SWD sub 20% > DP3 + Insanity > Mind Spike
Something to that effect. They will have to buff the 90 talents, but Celestalon has already said they will be in single target rotations so, that is going to happen I'm sure due to the healing component getting removed.