I have been looking for a sendero in 300rum for quite a while now, and remington has stopped making them, I e-mailed them to make sure that what I was hearing was true, but you can get the 700P in 300 rum, I ordered one today.
I also asked them the difference between the two, their response was that both the sendero and the P have the H.S. stock but the P has a tactical stock with larger palm swell, wider fore-end, and bipod swivel. The P also goes through a 21 point inspection for quality and preformance, and have a guaranteed Minute of Angle.
I will say that the response greatly influenced my decision over that rifle and a used Sendero.