I have worn my O'Neill Psycho II 3/2 through December depending on how cold the water is. I will tell you that my upper body will get very cold, but the unbearable part is in the shins, where you have 2mm on a part of your body that has little fat on it. You will def need a thicker 4/3mm+ suit come Jan, if not well sooner.
i have done it for the last few years, wore a 3/2 with a 3mm hood and 5mm gloves and booties but it is down right cold. even with a 2.5mm rash under it your going to be cold. Underarmor works but not as well as a real winter suit.
it really all depends on how cold you want to be