Cold temperatures (below 50, and especially below 40) ratchet up the kiting danger factor by several orders of magnitude. The colder the water/air, the more dangerous it becomes. Already slim margins of error become slimmer. A normally dangerous sport becomes even more dangerous. When is simply cold too cold? For me, the answer is mostly in the hands.
Numb hands in windsurfing is a bummer, but do-able. Many times have I returned to shore on a windsurfer with my hands too cold to move, or feel. (After they thaw out, the THROBBING is intense and painful.) In kiting cold hands are, for me, with my safety system, potentially life threatening. My hand(s) must FEEL the quick release ball in order to GRASP it and pull it. This is, critical, a threshold safety issue. Cold legs or feet or arms are a nuissance. Cold, numb hands are a serious liability.
Also, the presence of all that extra equip -- hoods, gloves, booties, etc. -- is generally more restrictive, to the body, the hands, etc., thus narrowing the safety factor yet further. Less visibility. Less manoueverabilty. Less dexterity = Less room for error = Less chance of getting out safely if something should go wrong (god forbid!).
Obviously, the experience of cold and "coldness" will vary from person to person, so everybody will need to make these judgments for themselves. Just be aware and vigilant to the fact that the cold air/water will place extra demands on you and your equipment.