I have, for many years, used a number of Wimberly and RRS plates in my RRS lever clamp. In all cases the Wimberly performs identically. That is not always the case with Kirk. Wimberly also officially guarantees RRS lever compatibility. For this reason I use Wimberly without any concerns at all.
I've never used a P5; I use L brackets for the same reasons Jon mentioned.
I think a P5 is a good tool to have in the box because it will fit any camera. It would be useful, for example, if you were early adopting a new camera prior to the availability of a custom plate. Or you just have a need to put a plate on a camera you don't own, or some other device. I've long thought about buying one just for the "tool box".
The biggest issue with the P5 would be any tendency for the camera to twist, which is the reason for all the custom bottom plates that are engineered to grip the edges of the camera. If not for that there would be no compelling reason to buy custom straight plate camera brackets.
I've seen very little feedback on the P5 so any comments about plate twist from either of you would be appreciated.