While Grimoire of Service/Supremacy sim essentially even with each other, as does Imp Swarm/No Imp Swarm glyph (if used with CDs), I have found that Grimoire of Supremacy (Wrathguard) and NO Imp Swarm glyph is the way to go.
I am confident I'm using Grimoire of Service and Imp Swarm at the optimal times during fights, and to fend off some comments that I'm just incompetent and aren't using them at the right times, I've linked the following logs where I'm #1 demo warlock on 3 of the 5 Mogu'shan Vaults bosses we've cleared (Damn you Will of the Emperor).
I still can't get Service or Imp Swarm to be a DPS gain. The only time when I use Service/Imp Swarm is when a fight is less than 90 seconds.
Just some friendly advice to demo warlocks trying to min/max like I am. Feel free to ask any other questions / offer any interesting tips that we might find useful!