Are you running 10 or 25? And who are you healing with? I would say that if you aren't having mana issues you may want to switch to the int/crit meta. If you prefer mastery over haste you may just want to pick up more crit, or...if you like bubbles so much why don't you just MARRY them?!
But 1:1 PoH to PW:S is the highest healing rotation for heavy damage phases if your mana can sustain it.
sim this, PoH->PW:S spam is only a few percent HPS over straight PoH at current gear levels, even with extremely high mastery. edit: I checked it again. PoH outscales PW:S, and haste outscales mastery even with 1:1 PW:S <-> PoH ratio. Net result is that PW:S->PoH->PW:S->PoH is now always an HPS loss for standing throughput, unless you are already running a high mst - low spi build. even then it's just barely more HPS than PoH spam (about a 3% margin at IL 398, with 2k spirit, rest mst>hst).
no, you can only justify heavy PW:S by precasting them. or if your heal comp is severely lacking on spot heals.
OP: If you're heavy mastery and -not- having mana issues it seems like you're using mastery to boost PoH efficiency. But really for mastery to be valued high enough to warrant stacking it you need in the neighborhood of 20% throughput from PW:S. In which case you need high spirit as well.
I just don't understand the benefit of a high mst - low spirit build, unless your HPS strategy revolves around DA. DA builds tend to top the meter when you out-gear the encounter pretty convincingly, or when you stack enough healers to trivialize content. I haven't seen it perform well in spike-intensive situations (progression).
I've had my mind changed a lot along the way, though, so if you felt up to explaining your reasoning I'd probably wind up learning something.