I would really like to see pictures of a 5-point belt used with a Boxster / 996 seat. Did they cut a hole in the seat cushion for the antisub belt?
I wouldn't trust any club that allowed a convertible (with no rollbar) to run with a harness. Scary thought. If you're investing in a fixed-back seat and harness (ostensibly in the interest of safety) then the additional cost of a rollbar extention seems a trivial, but necessary, addition.