I wear a 5/4/3 suit in the winter, and the 3mm in the arms is totally fine. Probably the only part that never feels cold. I'd maybe be worried about my legs in nothing but 3mm rubber in February though...
That said, the Cypher is a pretty solid suit. Fully taped out with a chest zip, so the water flow should be pretty minimal. I'd say the deciding factor is really the accuracy of the fit on you. If you know your size in Quik suits matches up nicely with your body, then you should probably be able to manage. Worse case scenario, you could beef up the thickness of your hood and booties to compensate the difference for a little extra toast.