When I'm tanking heroic 5-mans, my mana regen is pretty evenly split between Sanct, Divine Plea and Spiritual Attunement. I wouldn't say I instantly go OOM if I'm being shielded, but I definitely notice the difference. This is made worse when the gap between pulls is long enough to let DP drop. (I'm looking at you, Violet Hold)

That said, a lot of the time in 5-mans I can get away with casting consecrate only once per pull and relying on Seal of Command and Hammer of the Righteous to keep the mobs glued to me, and that's usually enough to compensate for the lost mana.

If you want more healing wear cloth when you tank. There are a lot of other mitigation priests and other healers use on tanks all the time. They just might not be as visible.

I would go along with Carl that DP can hurt you on far apart pulls.

Also remember you aren't really getting any procs off of caster mobs on your Blessing of Sanc (Also looking at violet hold), which can wreak unseen havoc on your mana pull.

VH pro tip for pally tanks - learn where the rats and spiders are. Run to one every other pull or so (most groups I run with, the Divine Plea cooldown is up after two groups) and hit it once (aka one-shot it). Run on. Divine Plea need never be down.