It's a good idea, but I think it would be OP. Holy Priests are already probably the best AOE healers in Wow atm, buffing sanctuary this much would be dangerous. It would go from what it is now, a situational spell which is meh in the correct situations and completely useless in other situations, to being a must-have in the correct situations and decent in other. I would assume holy priest stacking would ensue since, feud would become quite easy since PoH and CoH spamming would take care of overall healing while the sanctuary keeps the low HP guys safe.