The reason Heal gives D8s is because that's all it gives: HP. Lay on Hands gives AC, Soothe gives Saves v.s. Emotional effects, and Goodberry counts as eating food for survival. They are temporary benefits, true, but something that Paizo feels is worth the loss of dice. If we wanted to make Heal have the same dice (which is fine by me, but sacred cows and stuff), it needs to give something in exchange. Maybe temp HP with a duration and value equal to the spell level?
See this is flawed though because Heal/Harm DOES do other things too. In addition to being a single target spell, in addition to being able to be cast 3 times in a round, in addition to being able to heal AND damage all targets within 30 feat it heals/deals more than all the other heal spells. And that is BEFORE being specced into healing.
I still advocate that the issue ISN'T Channel Energy. That was hiding the real issue. The real issue is Heal. Either we have healing parity or we don't. Druids rarely get brought up in the healing discussion because they have access to Heal. They can cast Goodberry in the morning and hand them out (like an Alchemist) and still prep Heal to use during a fight. That is great versatility.
The reality is of the healing classes Cleric is the only one really given class feats to support healing. I think if the other classes had access to Heal - or a spell that has actual parity with Heal - and had additional class feat options to spec into healing specific, things wouldn't be so bad.
I made a suggestion elsewhere that maybe Soothe should get an effect like Inspire Heroics where you take an additional action to do a performance check and based on the result Soothe affects additional targets (the amount of healing is reduced to your casting stat). Maybe there is a class feat that allows Soothe and Soothing Ballad to do d8 heals. This starts to create parity. Clerics are still at the top but the distance between the two aren't as big and a group doesn't feel like picking Lem (who never gets picked anyway) destroyed the group.
I'd love to see Alchemist get some way to do a group heal. I keep saying a Healbomb would be fun. It heals the primary target an amount and those around take splash healing. Or even healing darts that obviously would heal less than an Elixir.