The OR Soul or the NPX Lucifer (harder to find) are probably the best kiteboarding drysuits around. You are not going to be any warmer than you are in a quality wetsuit though. + the real problem in cold weather/water kiting is your hands!
A membrane drysuit is more comfortable to wear but I find it actually restricts movement compared to a well fitting, good quality wetsuit. Particularly if you wear a bouyancy aid or impact vest over it. The Neoprene kind are pointless as they don't do anything well.
Even a drysuit with a big hole in it does not have negative bouyancy. There will still be trapped air in it so you will always have more latent bouyancy than if you were in Hawaii in your boardshorts - even then you would have neutral bouyancy.
A good drysuit is a good option but a good wetsuit is just as good. the only thing that really sucks is getting out of it at the end of the session - particularly if you do it next to your car in a hail storm in a car park.... dry suit has it beat there. but a decent wetsuit is half the price, just as good on the water and a much better choice for surfing or SUP if you do those sports too.
Other popular options if you are in the middle adged, middle income section of UK kitesurfers, are spending 20 grand on a camper van or 2 grand on a winter holiday to the caribbean for a couple of weeks in order to avoid being cold.
The sea temp is still 9° in the Hebrides - Speedos weather