As far as I can work out , the +P is not a chamber modification other than a longer throat to allow extra powder to be burned .
So my understanding is that you will be able to fire all 338 Edge rounds in it.
This is the developers thread that explains it :--
338 Edge + P now available from Defensive Edge
If it were me buying one I think I would like a throat depth matched to a certain LR bullet and stick with that but I guess they are going for max velocity to stay super sonic as far out as possible . Seems to be working .