Discipline regen scales incredibly well with spirit - much better than the other healers as rapture on cd roughly doubles the spirit -> mp5 conversion. Most of the "help me I'm running OOM" problems are temporary. I've already started flasking / eating int food on my priest, but that's also because even in a mana limited environment the correct strategy is not always "get more spirit" - in some cases it is to get more intellect (when taken at a 1:1 ratio with spirit) so that you don't have to cast as frequently as your heals are more powerful - and by extension also more efficient.
Also, both Stone Guards heroic and Gara'jal (normal and heroic) have mechanics that greatly increase your mana regeneration. Mana shouldn't really be a big issue on either of those fights.