Pretty much you're going to be OP once you've gotten EXP Share and haven't turned it off
The entire time, but at the cost of effort points and what not. You won't get the bonus +1 stat or +2.
For Gengar if you have MegaPunch your ATK will get more effort points the more you faint pokemon with it, but this is only effective if it has a decent nature and wearing the Black Belt I think.