Andyy.
All aircraft design is a compromise. Optimise for one particular flight regime and there will be a cost in another part of the envelope.
Just out of interest, what would you consider to be the features of a wing that is optimised to operate at low level? What would you then think are the negative points of such a design when said wing has to operate at high level for other tasking/transit?
Dont get drawn into MAD (at least in the traditional sense). The technology of submarine detection from aircraft and the design of modern subs to avoid detection by such a sensor has moved on considerably in the last decade.
To state that P-8 "will have to deploy (I think you mean sono) buoys from higher altitude" is 100% wrong and an opinion not based on anything.