Interesting multi-role potential... Note that the P-8A also has ALQ-218, a version of the Growler ESM, which would do some decent SIGINT with the right operators on board.
However, if you look at the USAF's plans to replace Joint STARS, they are based on a large business jet. 10-15 kft more altitude is a big advantage in terms of terrain shadows once you get beyond the littorals. However, the Boeing-Raytheon team is very well placed to win this - the main change needed would be a smaller antenna.