have u killed the last couple boss's yet? like empress, protectors etc? Im not saying healing them with 8k spirit isnt possible, as it obviously is... Im just saying personally i felt a lot more comfortable being at 10k+... but my raid doesnt have a shaman giving me mana totem, where as maybe u have a shaman and preist in ur raid providing hymm+totem.
Like i said, it was jsut a guideline of wot i personally use... the point was more that the end couple boss's require a lot more healing than the 1st 5 and the 2nd lot of 5... I noticed a big jump in mana usage hence the increase in spirit
Ive even droped down to 9.2k spirit for our empress farm kills now, but thats because as a raid we are more efficient at killing it, so less damage, better mana management, etc etc