Hi Granyala,
Yes, I ranted a bit about that in FAQ section.
That I can tell, it's not a dps increase to Flay, even for one GCD. But if you look at the way the spells work, it seems like that was Blizzard's intention for the spec. And again, if you look at how the class trinket works, it provides even stronger support for it working that way.
It seems that with enough haste, the class trinket might line up in such a way that we would flay for one GCD just after a Devouring Plague. I don't know exactly - I'm not on PTR. But if it works out like that, then I guess there was some internal design built around us playing that way.
IMO, CoP shouldn't use Mind Flay ever, only Mind Spike and Insanity. However, my opinion may have no bearing on what ends up happening with the spec long term. Bliz might make some change to the spell mechanics - besides an awkward trinket - in a future patch that makes "flay-weaving" mandatory.
However, one interesting thing to me is that they've nerfed Devouring Plague by 12% in the next patch. Previously, there was a breaking point where L90 spells like Cascade were worth casting in single target whenever DP was ticking. The nerf to DP will push that breaking point higher, past the damage that Cascade can do against a single target. This is actually even MORE in favor of spiking off the DP dot. The less value DP has, the less important it is to maintain its residual dot damage. Also interesting is that the nerf to DP is a minor nerf to DoTWeaving, which already will take a massive hit with the loss of the t17 4p set bonus. I might even go so far as to say it would kill DoTWeaving as the primary single target play style, in favor of Mindbender. Insanity would then get relegated to AOE fights (where you would still DoTWeave when there's a break between AOEing).