If you only change a particular part like claws or teeth, the stronger it will be. If he is a full cat, he might be able to run quick or have some heightened senses, they won't nearly be as strong as if he just enhanced his eyes or something.

Wait does this mean that he chooses to go out with just fangs at some point the teeth are extra powerful? Or particularly good hearing for ears only? And then when he's full fox he's just useless furry?

I'M ASKING JUST IN CASE THIS IS TOO OP AND THAT'S NOT WHAT YOU MEANT.