Shaman wiki gives bad advice.

[Pyrion]

A larger HP pool will not give you more mana regen. Not at all. Not a bit. You get more mana potential, but only until you use it. This is important and a lot of people do not understand it. For a shaman the health regeneration rate can be directly converted to mana regeneration. The max HP is totally worthless to mana regeneration (although its nice to have for obvious reasons).

A large pool of HP means that canni costs less HP %, yes. But how does that help mana regeneration? Your heals don't heal HP %, they heal exact amounts of HP, no matter how much you have. If you would have CH like a cleric that would be a different story.