Like I said I have not had any problems with the valves of my Naish X2s. They are softer than valves of ARX. I don't know Sligshots. -15C = 5F is cold if there is hard wind. But 10-14 knots with 20m2 X2 is nice if it is sunny. Just wear right kind of clothes that you don't sweat and get wet.
Especially, if you are not near sea or open water, then air is dry and moisture don't increase the wind chill.
Foils are one solution but if you like inflatables you can still use them if you have good valves. In -25C = -13F also I would select a big foil.
But this is just my experience. Here the winter is long and dark, so you have to do what kiteboarder have to do
Kids, do not try this at home.
PS: Greetings to Canada!