kartchy - I have two catchers platooned who have almost the EXACT same rating (PC 49 vs PC 84). Over one full season, the 35 rating delta resulted in an ERA delta of 0.43... which is almost EXACTLY the difference explained by MikeT's formula of 0.125
35 PC delta/100=3.5 3.5x0.125 = 0.4375 expected ERA difference. Mathematically, we hypothesize that you should see about the same, assuming no other defense (etc etc) has changed.