Ditto Per. I also use a "regular" Gul suit, 5/4/3, not even especially dry. When it's cold (meaning aroun 5 deg C) I also use "skiing underwear" instead of bulking up with more neoprene. To that I use 7mm Atan booties. Super comfy and warm and still better handling than many regular 3-4mm booties. Still (with my toes and fingers destryed by cold skiing) feet and hands are the problem. The DaKind cold water mitts are good, but fingers still set the limit for me.
The real thing to consider when sailing in the cold is what you do when something breaks, or you loose your gear. You have to have a plan for that. I try to stay as close as possible to where the waves break so that I can let the waves wash me ashore. If its 5deg C air and water, 40 knots and strong currents, you will have a rather slim chance of making it in on your own even if you're only a few 100 meters out. So be prepared and use high quality and fresh mast feet and stuff.
Below is a good day in Sweden some time in january this year.