Since you're in tier 4 now, I absolutely would not recommend Grace healing (typically done with a shield now) for either scenarios or oRvR. In Pug scenarios it is currently extremely tough for even a rank 40 Grace WP. It really a solid group of team mates around it to be able to start to perform.
If you're happy levelling purely from scenarios and RvR I'd definitely look into salvation. Salvation seems to be the most effective PuG healer. Around your rank i'd probably run something like this:
http://www.ror.builders/career/warrior- ... ,4151,4173
In organized warbands WP is generally expected to run the AP tactic, but frankly in PuG situations this is not optimal. You don't get as much bang for your buck as PuG players are not that good.
I'm not sure about the state of DPS warrior priest but if you want to give it a go then don't let anybody stop you. There are a lot of raging jackasses that whine because you're not playing backline healer spec to healbot for them so be prepared for that.
Almost every scenario you get some guy like ''omg look at all these luldps healers'' like a broken record. I even get it when trying to test the new Grace changes, as a legit healer build.