So something like: Msp + MB until 3(5) Orbs followed by DP + Mindflay Channel. But that's just theory and it's actually pretty much impossible to tune single-target with Mastery as the single target stat:
DPs damage is not increased by Mastery.
Mindflays damage is only increased by Mastery.
Mind Spikes damage is increased by Mastery, but has high base coefficient, plus CoP multiplier.
For the above rotation to work DP DoT + MF has to always be better than spamming Mindspike. DPs instant damage has always to be better than Mindspike, etc. Bash me for taking current numbers: Even at entrance level stats Mindspike currently beats DP in damage. So currently CoP would consist of one button to press.
As long as Mastery does not increase DPs damage or Mindspikes damage is increased by Mastery there will be a point when casting DP is no longer worth it (or T90 talents). Only change that would prevent this from happening is to remove Mind Spike from Mastery.