I agree Soul Siphon is not the best in this built and Coax to Darkness is better but I was showcasing Affliction enchants.
Growing Corruption is a two for 1, you don't have to spend the global cooldown to cast Corruption. Wild Wrath may be better so you don't have to keep casting insect swarm and you can use the talent Careful Aim from Marksman Hunter to get INT to Ranged AP. This is especially true if you are using the Everlasting Affliction talent to keep Corruption up permanently with your Haunt. Again, I was showcasing Affliction enchants.
So you cast Insect Swarm and Corruption and keep them up with Haunt on cooldown, wrath when needed with a steady shot-->wrath if your Swarm is about to cool down. In between wraths you are casting Devouring Plague and Unstable Affliction and Curse of Agony.