"What's the cam plate do for performance?"..........While I cannot answer that question directly, I can indeed express my dissatisfaction with the move that H.D. made to..................."parent material".
If ya don't know, this means that while (in the past). there were such things known as "cam bearings and/or bushings, they HAVE BEEN DONE AWAY WITH!!!!!!!!!
parent material means NO BEARINGS/BUSHINGS at all on the outer side of the cams. Just another "CHEAPINGING" move on the part of H.D. in my opinion.
Per your question, if you opt for the "cam plate" SE style, you finally get a bronze bushing in the cam plate to support the outer end of the cams.