Sability curves are created by evaluating the distance between the buoyancy vector and the gravity vector. My physics is a little rusty but I wonder if there might not be an additional factor affecting the tendancy to capsize. When a typical kayak starts to tilt it tends to go lower in the water because the edge of a kayak is typically not as tall as the kayak is wide. When you are floating on the edge of the kayak you are thus floating a little lower.
It seems to me that this factor might be an indendent factor acting to encourage capsize. As you tilt, the center of gravity is falling which might add energy to the capsize. Conversely, as you right yourself you also have to actually increase the height of the center of gravity, push yourself up so to speak, as the kayak will typically be floating higher in the water when level than it was on its side.
If I have worded this clearly enought to make any sense, can anyone clarify this? Perhaps this is accounted for by the vectors themselves and is not an independent value.