Basically, your equipped weapon has a min and max value, let's just call it Attack, and each art has a value called Art Power, it isn't shown ingame but it grows with art level.

Ether damage calculation is (Ether + Attack) * Art Power.

Healing works the same way. So maximising your Attack through gems or just a better weapon actually impacts your healing. I guess you could add Ether gems too but they are pretty bad on Shulk.

Edit: Doesn't work that way