It is still VERY possible to max soul siphon without Siphon Life on the target. Most raid comps include at least 2 Afflocks which will result in: [(CoA + Corr + UA + Haunt + SE) * 2 = 10]
There is a VERY good chance if you roll in a smart guild you will have a Meta/Ruin utility along for the ride, so I am not crying about the loss to 1 debuff, as the synergy between all warlocks will cap soul siphon anyways, given they aren't baddies!
Just tested this on the PTR, Demonic Pact still doesn't take the spirit part of Fel Armor into account. Really disappointing.