Let's say the tank manages to increase his dodge amazingly with 10%. This would cause roughly 10 less hits or 280k less damage. Looking at 9 million total damage, this means around 3% less healing required in total. This is not a lot of saved mana for the healers, barely noticeable.
Actually, the mana cost for a healer to get a small increase in HPS is very significant.
If you download treecalcs101204.xls from the resto thread, and don't touch anything, you'll currently see on the 'Main' tab:
239s till out of mana
5.3% of healing is from Nourish.
I'm going to change all of his Nourish casts to Healing Touch casts. The spreadsheet doesn't directly support that, but I'll go to the "Heals" tab and copy the HT column to the Nourish column, and that has the same effect.
Now on the main page we se
26432 HPS: a 9.5% increase in healing.
121s till out of mana: you'd better kill the boss twice as quickly.
13.5% of healing is from Nourish (aka HT), roughly a 150% increase.
To get even more healing he'd have to change his spec (to increase Regrowth crit, so that Regrowth HPS > HT HPS), and start replacing HT casts with Regrowth casts, for much, much worse healing efficiency.
Your healers have damage levels that they can support very cheaply (Druid: Rejuv+Rolling lifebloom+Swiftmend+Clearcast HT+Nourish). Increases to that level are extremely expensive (meaning not sustainable).
Edit: The numbers above included some raid healing. Single target, that particular tree can
Heal 10-12k HPS forever.
Heal 19-20K HPS for one minute before going OOM.
Cannot exceed 20k HPS for more than a few seconds.