That was my first reaction too...yet another thing to micro. Insta-heals really need to go away (although they may exist as a crutch for the AI that doesn't know when to retreat).
Replacing insta-heals with an extra move (burn a promotion to get all your movement and attacks back, but no healing) could keep the good aspect of insta-heal, which is saving a unit's life, and have less of the bad side, which is unstoppable rampaging horsemen.
I don't think other GPs need a buff...it's the scientists that are out of whack. They are the easiest GPs to acquire and they are way too strong. I'd go with either (or a combination) of the following:
10-turn delay between lightbulbs. Great Artists can't chain-bomb; I think delaying scientists like that would work well too. You'd need to space them out instead of saving them to blow through an entire era instantaneously.
Partial teching. A GS should not necessarily finish an entire tech, but should give you a certain amount of beakers based on your era/the game clock/your worst unresearched tech/etc. Great Engineers work this way.