From my understanding, spriest/warlock is second best though far easier to pull off. The strongest is rogue/spriest, but harder to pull off.
I'm not sure, but I think UA would be better than SL/SL for spriests just because then you've got two classes who aren't "pure" dps - one is a lot of utility, one is survival., and neither is the overbearing amount of damage of a rogue, warrior, or frost mage.