Yep old akro, Norfolk island is the one that comes closest to a lifejacket being useful and yet only half the occupants of the aircraft were wearing one when it ditched according to the ATSB report. All survived so you do have to wonder did the lifejacket add that much.
In smaller aircraft the impact with the water is much more likely to be survivable and so life jackets do make more sense. The kinetic energy in an impact at 40 knots is significantly less than one at 100 knots.
The coroners report into the 1998 hobart Yacht race has frightening reading about the real world performance of of lifejackets like what casa mandate.