The reason I'm not that impressed with these calcs is because I could post the same thing except with Gengar instead, which isn't even ranked on the viability ranking thread.
Like literally, Gengar has 5 less sp atk, and could run Nasty Plot, Shadow Ball (over Dark Pulse), Focus Blast, and Psychic/Sludge Wave. You even get STAB on Sludge Wave.
I concede that 125 speed vs. 110 is significant, and that 70/90/90 bulk is a bit better than 60/60/75, (albeit Life Orb recoil makes the modest bulk increase less significant). I suppose you could make the case Dark Pulse is more spammable than Shadow Ball due to lack of immunities, but those moves are still pretty comparable.
Like I said before, I don't like theorymonning, but considering Gengar isn't considered viable, it seems wild to me that people think Darkrai is so laughably broken it doesn't even deserve to be tested.
Edit: fwiw Gengar even gets Knock Off like you mentioned for Darkrai too