If he's played the guy at C a bit during the season, I don't think any complainers have a leg to stand on.
As far as ERA goes, MikeT23 did some analysis a while back and found something like a .125 increase in ERA for every 10 points of Pitch Calling ability, Of course I would think that, as with all rating effects, the closer you move to either end of the spectrum, the more of a difference there will be. I don't think Mike would have tried any 10 PC guys at C during his study, so i'd expect a bit more than a .125 difference when going from 25 to 15 and from 15 to 5, but that's just a guess.