i really dont see this affecting to much, seeing as i usually have never down ranked when healing in Sunwell or BT, especially since there is a enough mp5 gear out there to get 2K+ healing + still have 200mp5 unbuffed while casting. my t6 shaman i use when raiding has 2022+healing and has a 196 mp5 unbuffed, so while full buffed, pots/ointments/food/water shield i si nearly around mid 400s, which is plenty while casting.

Especiall for fights like RoS/Teron/Blood Boil im usually hovering around 50% mark for mana.