The first post is slightly outdated. The optimal "rotation" (used by the BiS profile included in Simcraft) for Demonology switches to the Felguard (by hardcasting it) right before using Demon Soul (and Metamorphosis) and then using Soulburn to switch back to the Felhunter right after the Felstorm of the Felguard has worn off (while in Metamorphosis). This will let you gain the benefit of the stronger Demon Soul while at the same time let the Felhunter contribute its incredible DPS (about 8k for the BiS profile).
On the same note, the scale factors have changed with the changed playstyle. If you are syncing up Metamorphosis with Demon Soul and your Moonwell Chalice (which you should), Haste looses a lot of its value, because you will not be able to use additional Impending Doom procs after reducing the Metamorphosis cooldown to 2 minutes. This can be seen in these two reports:
Delaying vs
Not Delaying. For some reason, the BiS profile does behave differently. It makes virtually no difference if you wait for the Trinkets/DS to get ready or if you just use Metamorphosis on CD, the scale factors do not change relative to eachother. My hypothesis would be that this is because the BiS profile has a lot higher Mastery:Haste ratio than I currently do (BiS 1.49*, me 1.15) and that 1.5 is the magical "break even ratio" between Mastery and Haste. The only haste threshold I am above and the BiS profile is below is the 10th Corrution tick with Bloodlust. This should not be a big deal, considering it will not save us even one global cooldown in the 40 seconds that bloodlust is up (Simcraft reports:
With,
Without).
So in essence: Once you obtain the Moonwell Chalice, sync up Metamorphosis with your Trinkets/Racials/Demon Soul and go for Mastery. I think.