It's kind of funny what happens when a person has all that time to think while the epoxy is curing. I'm still overbuilding but that's just to make me happy and doesn't reflect boats breaking, just an anxious mind. For the money you can't beat multiple layers of 4oz glass strategically placed. Spending 4times as much for 4oz. s-glass on the "football patch" is a fine way to indulge in the building fetish while getting to know J.R. Sweet and all their products.
Still how and where you place your kayak is the greatest factor affecting durability. A kayak I made for a friends rental/touring company had heel wear spots in the cockpit that went through three layers of 4oz and half way into the first veneer of 4mm ply, and the rest of the boat had none of the stress fracture problems that are being discussed in this thread.