Randy, if you're really looking to get some toughness for rocks putting on a softer coat of slippy graphite epoxy will make the kayak easier to slide around but it won't make the kayak tougher,,,imagine that the kayak is bumping back and forth on the same spot as opposed to sliding past the rock. With each bump by the same rock,in the same spot the only thing beneficial about that graphite epoxy is how thick it is. In that case it doesn't matter if it's graphite or not. A thick fill coat of epoxy will provide more protection to the glass than a thin one,,whether it's graphite or unadulterated epoxy. Basically the graphite is a one shot deal for abrasion only. As far as the laminate withstanding the impacts that's a function of the weight of the glass.
So let's say you put on a thick coat of slippery epoxy/graphite since thick lasts longer than thin. With a big enough impact the thickness of the fill coat,,or type doesn't matter,,it's back to the thickness of the glass skin resisting breaking/denting. Imagine that thick fill coat was instead used to wet-out and fill 4oz s-glass? There you'll have greater resistance to dings into the wood which tear into glass AND greater abrasion resistance.
There's something kind of "free lunch" like about being able to paint on a few ozs of graphite/epoxy. Putting on a 'football' of 4oz s-glass may weigh 8oz and a thick fill coat of epoxy/graphite may weigh 3-6oz. Dollars to donuts that extra layer of glass will do more for landing/sliding on rocks than the graphite and you won't notice the weight.