Im mainspecced Shadow but with offspecc as holy, and its just fine np at all. Most fights I've healed im doing the same HPS as our MS Holy pala. Healed lei shen yesterday was 1-2k under the hpala with disc priest topping meters.
My guild have told me several times that it would be nice if I went disc as OS but I just told them no, cuz I don't really like disc as pve healing.