I would go Full Demo, then, if that's what you really want.
Quite frankly, you're looking at every teir in Affliction Simply becoming MORE tempting... (After Improved Corruption theres Improved Lifetap/Soul Siphon, then Amp Curse and Fel Concentration, and then you might as well be a friggin DrainTank.)
If you really want to embrace the Demo Spec then, for now, Pick up Improved VW, and get your points down into Manafeed (rank 1 at 35) so he can actually keep up with you. If he is tanking so well anyways, 2 seconds added to casting time of Corruption ain't gonna be what is slowing you down -- his mana is.
Go back for Corruption later, or if you're into PVPing then you definatly need it -- but if you're PVPing I wouldn't go Demo at all right now.
Just my 2 cents...