Great plan PN. Unfortunately, being able to place sonobuoys close to the tgt is still required. This drives the requirement for sonobuoys which can be guided or can guide themselves. Your weapon system relies upon being able to guide itself or be guided to the the ideal splash point.
Such systems are being developed by the USN. The cost of development is significant. If there isn't a problem with the P8 at low level then why do this?
If the UK acquires the P8 I suspect we would have to do a significant amount of work to certify a UK weapon as the current USN weapon is not acceptable under current UK rules. I also understand USN sonobuoys are not interoperable with the RN and othe NATO ac.
The MTOC which supports the P8 is massive and manpower intensive. Another hidden cost.