Go with two warlocks. You defintiely gain benefit from that first priest, but much less from the second. Spec the priest with misery and shadow weaving to give the warlocks the most benefit. Gem up your priest with resilience and stamina gems, make him as much of a tank as possible because shadow priests die very fast otherwise.

All priests... well the main issues are they go oom and have no way to get it back. In long fights my priests go oom long before the warlocks and I'm pretty much just using the priests to lead the warlocks around at that point - they don't cast anything. Second, their only nuke has a cooldown, so you can't spam it and if you get lucky enough to get a bloodlust in your group it doesnt you very little good.

I'm all for grouping a priest with a warlock group, and I kind of cringe at warlock only groups (no dispell, no mass dispell, no resz, no shields, no misery + shadow weaving +15% shadow damage, etc.), but with multiple priests you just get diminishing returns