OK, as if I haven't had enough to say on the topic of P-lips recently, let me wade in with my 2c worth:
For me the P-lip is a great clencher, though I don't clench much except while driving.
The down side for me is that at my clench spot, as it were, the relatively narrow, upward facing smoke hole tends to really concentrate the smoke on a small area of the roof of my mouth, leaving a tender spot. Hence my mod referred to on previous post.
Generally, I think it is all down to the fact that, apart from simple personal preference, we all have unique 'dental landscapes' and mouths. So in terms of clench comfort and/or smoke direction/draw it works for some, not for others.
I may just mention that one of the members of our pipe club here in sunny (at the moment) Cape Town is a medical doctor, and he has also raised a question regarding the desirability of concentrated smoke in one spot from an oral health point of view.
Well there's my 2c (and change).
M