I had a medieval V1 crank pulley and the plate with the teeth for the crank sensor sheared off and broke on me. I did notice the difference from the lightweight pulley, it wasn't an underdrive though. I have a medieval PS pulley as well but never installed it since it's not worth risking damaging the pump to replace the pulley.
I got a decent gain from lightweight and underdrive crank and WP pulleys in my Bullitt, would love to have a complete set for the MSP!
