---------- Post added 2012-05-14 at 11:26 AM ----------
You get 33-40% damage mitigation, but almost all heals are reduced by the same amount by being split between the pet, except for Dark Regeneration, which heals both the pet and the Warlock.
Hmmm, but this can also be bad for the Warlock. If you are fighting another lock, all he can do is to DoT up both you and your pet to do double DoT damage, unless of course, he is using a phased Imp or permanently invisible succubus (though who would?).
In a sense, Sacrificial Pact's 200000 HP bubble prevents healing being mitigated, or additional damage dealt to you through double DoTing or spells like Harvest Life or Multi-shot, which do multiple target damage.