MAD is a great confirming sensor, but you (i.e. flight crew) and the maintainers have to stay on top of it with comps, etc. I can't tell you how many times I heard "madman, madman, madman" either in the WST or at AUTEC, followed by my "roger, weapon away," all leading to a spot-on torp drop.
I don't know how it's done now, but I can assure you that there's been plenty of ASW done at 20,000 feet. My 3rd grader uses my old TACCO clipboard to do homework on the couch while watching Hannah Montana. The clipboard has a buoy drop/OTPI error chart laminated on one side, with altitudes from 1000 to 25k feet and windspeeds up to 70 kts. Plot stab was definitely more difficult at altitude, but we still tracked 'em. Of course, MAD was useless up there, but I don't think the P-8 will be staying high to drop torps.
I know the Nimrod folks had some way of picking up their buoys on radar, thus plot-stab problem fixed. I can't imagine that the P-8 won't have something more sophisticated then OTPI and MOT every single buoy. Maybe MAD is an antiquated tactic, but the last time I saw a P-3 MAD system, it had a paper gram.

Brett