P means professional. They are stronger then amatuer dan. You could grade at 9dan strength (depending if you think EGS, AGA, or KGS ranks).

OPne thing to note, is that handicap stones work differently with professionals. A 1 dan professional isn't 8 stones weaker then a 9dan professional! At the most 2 stones, but a couple of points stronger goes a long way a that level. I think the biggest difference is being able to withstand the pressure of the tournaments.