Not saying that the whole combined capability and joint servicing we would benefit from in joining the P8 MMA programme would not be great, but if the Kawasaki P1 is a bespoke maritime aircraft, with the undoubted high grade mission kit the Japanese would use (also compatible with previous P-3s), then it would get my vote.
The thought of a 2 engined commercial airliner, designed for high level cruising, albeit heavily modified, operating in a maritime environment, would not fill me with much hope for its long term durability. Having to achieve accuracy when dropping sonobuoys means low level, which means corrosion.
I would love to get a look at the P1, and wonder how much of a multi mission development is planned, or is possible.