it's been well-established that it is not hypothermia that is the primary killer in cold-water capsizes; it's Cold Shock, with the companion involuntary Gasp Reflex causing the sudden inhalation of water
Do you have any references that show this? My understanding is that cold shock is more prevalent with colder water, and at water temperatures we see here in California (ocean temps vary between 50 and 60) it is not that prevalent.
I did a quick search (including this Sea Kayaker article at http://www.seakayakermag.com/2008/Feb08/cold-shock.htm
), and they do say the colder the more it occurs, but can occur (though they make it sound like rarely) at water temps as high as in the 70s.