-- Last Updated: Sep-09-09 1:29 PM EST --
WHere are you getting this "rating" that neoprene as a material is only good to 50 degrees? Many people don't wear even fingered gloves at that water temperature.
Consider that you have a specific condition that makes your hands intolerant of cold. A solution that's warm enough for you will be very conservative for the majority.
As far as Goretex mitts go, even with long cuffs, if the cuffs don't have waterproof seals (as found in a drysuit), the inside will be soaked after a full immersion. GLoves made for snowboarding aren't going to have waterproof cuff seals.
Neoprene retains its insulation when soaked.