Slow down battles? Sure, as long as the rewards are worth the time spent.

We don't need to create a system specifically to combat powerleveling, nor will they do so. If I recall correctly Yoshida's official stance on the matter was that if people wanted to help their friends, they were welcome to do so even if that meant powerleveling.

You can't stop it from happening, people will find a way no matter what. It may be slightly more inconvenient but it won't go away completely unless you design a system that punishes ALL players, even the legit ones.

The method of gaining XP now is fine, the pace is a little too fast but the actual system in place is fine.