I actually think the Abyssal Drain would be a nice and easy change. It would not affect 8-mans at all and make AoE tanking a bit more interesting, I don't think you need to reduce the potency either, since it is already a DPS loss over Flood.

200% life leech is nuts, so no. But 400 potency per GCD would be nice and help the healers top us off. The invulnerability should last full 10 seconds too. When you reach 1 HP you should receive Walking Dead and something like Undying Fortitude. The latter always lasts 10 seconds and makes you unable to die, Walking Dead is what kills you if it's not cleansed.

Blood Weapon should have stacks.

And I'm all for reducing the oGCD bloat by whatever means necessary.