Underneath view of the P8
Looking at the picture posted by Diabolo Rouge, note how far aft the bomb bay is - surely most other designs try to get it as close to the CG as possible!
It is also noticeable how small the internal carriage is compared to the MRA4 - it looks (ahem) considerably less than 1/2 the length.
I would also bet that the P8 design isn't capable of carring a full weapon load, sonobuoys and full fuel all at the same time, without having some sort of active fuel management system to maintain CG limits.
As a final observation, why does it need a torpedo that can be dropped from higher altitude, with guidance during descent to ensure splash point accuracy - is this actually because the aircraft hasn't got good enough handling to operate at low level?