I have found the opposite to be true. When I pull larger groups in heroics I have more mana at the end of the fight. If I over pull, intentionally or otherwise, I generally finish the fight with 100% mana. I buff Sanctuary, SA is glyphed, I have talented Mana Spring Totem down and my warlock provides replenishment. The summation of these probably explain my experience with bountiful mana. I generally judge SoR on trash, or maybe Light.