well i see holypriest as a tankhealer about as much as a druid and a schamy.
not optimal, but since cata defenetly doable.

but critbuff will defenetly help the holytankhealer quite a bit, not only becuase of hw:s/fh/fh/gh combo, but also because of the echos. fast ticking hots are allways nice on tanks. especially when they translate potential overheal(crits) to heal (echo) :-)

speaking of substainability and assisting
ever tried rolling renews on 3 tanks (or people that take regular dmg) and letting PoM jump around?
you can do tons of really efficient heal by doing so, and spammhealing heal on the tanks (and gheal/hw:s if tank is going under 80% or so)