I know I'm rusty on the fine details of shaman healing, but how does keeping 100% uptime on RT equate to a 100% uptime on AH [assuming the shaman has "normal" levels of crit for a raid healer].
iirc there were two ways of gearing for resto shamans in cata, one of them was stacking crit, so most of the time you are free to assume that a given resto shaman has more crit than '"normal" level of crit for a raid healer' (resto shaman who heals in my runs has round 20% crit unbuffed).
they also get ah procs from chain heal, which they like to use on melle if they are raid healing, so chances on having ah on tank even if shaman is raid healing (assuming tank stands somehow close to other melle ofc) are quite big
but tbf i dont remember any issues with keeping tank up against physical dmg, and raid doesnt really get much of physical dmg, so i wouldnt overestimate it.
havent tried neff hc yet so maybe then ill change my mind and get inspiration if at some point tank healing will become more challenging.