Without running the numbers, .25" seems a lot more reasonable than 2.5". I know a 40' rod headstay stretches about 1" going to max load. If it were 2.5", I think you'd see a lot of thermal rigging failures.
Another place where you see thermal problems is with expansion of aluminum frames for hatches and lights. They expand a lot more than the fiberglass deck, which will fail the seal if it's not thick enough.