They already have a tree with SLAs, so I don't think they should have another with more, because I can tell you I would be the first to simply pick them all up from both trees. That being said, if they do get SLAs these are the ones I would pick:
t4: break enchantment
t5: holy aura (after it is fixed and working correctly, I never heard that it was so I assume it still isn't)
These two spells are useful to the role of a warpriest that I envision, where you are playing to the strengths of your class while bashing in heads. Break enchantment gets rid of alot of annoying things, most notabley enemy blade barriers. Holy aura both buffs you and blinds enemies, having all enemies blinded is like casting displacement on all your allies, it keeps them alive better, and also enables sneak attack damage, making it a very nice thing to have.
I would not do 3, simply because you end up filling your slots with useless spells already and having even more spots filled with useless junk seems wasteful. If you have to give them 3 I would give them displacement. It is self only so you are not taking away from someone else's buffing power and, well, it is displacement.
Baring those I would look for spells that they have not added to the game yet and pick some from there (and add them to the game at the same time.)
But honestly I think they would be better off doing something other than SLAs. I really Like the idea of mass cures on vorpals, but think it would be much cooler if it is on a crit. Having it ona crit makes weapon selection more important for a warpreist than it otherwise would be. I would have it be a mass cure light on a x2 crit, mass cure moderate on a x3 crit, etc. This way you cn have the dual scimitar guys from the undying court tossing out tons of small heals, and then you could have the guy who splashed in some fighter and is bringing the heavy picks dishing out the occasional free mass cure critical, and over there you have the bladeforged cleric who is cleaving through foes dropping multiple mass cures. The coolest part though is you might even see some weird combinations like rapier/heavy pick for someone who wants to have frequent small cures and occasional big ones.
I would have a line of enhancements that adds to-hit and damage upon spell cast, and adds spellpower upon weapon strike. Warpreist should not play as a fighter with backup spells, it should play as a cleric that bashes in heads in the name of his deity.
Have an aura that boosts movement speed by like 15%, same bonus type as barb/ranger sprint boosts. This allows the rest of the party to keep much closer to those guys and get to the combat sooner, without leaving the party behind.
that is my two cents