One thought here:
A pitcher who is pitching "over his head" will certainly throw more pitches per inning and likely will throw more per at bat and plate appearance. Thus, given a certain Sta/Dur combo, he will log fewer innings/ab/pa than he would have at a more appropriate level. It's not clear to me whether "playing time" for pitchers is represented by pitches thrown, IP, AB, or PA. If it is pitches, it should not matter if he is over his head. If it is AB or PA, then it would matter some b/c of higher pitches/ab and likely more walks. If it is IP, then it could have a huge effect.
In fact, now that I think about it, if your pitcher is getting bombed badly enough that he can't get deep in his pitch count, then it doesn't matter how they count playing time, it will have some effect. Even with a call bullpen setting of 1, a guy will tend to throw fewer pitches on average when pitching badly than when pitching well.
This doesn't seem to factor in for positional players, as they don't get pulled from games in the same way.