Slight gain in bhp but slight loss in torque, also when the cranks are designed it is taken into account what wieghts are on either end, so if I were to even think about a lighter pulley I would have to put on a lighter flywheel to 'balance' the weight loss, and the loss of torque would far outway the gain in bhp for a road going car so I wouldnt tamper with them at all, I did put a lighter pulley and flywheel on a mk1 GTI once but that was when I was young and stupid, and it was only after doing it I realised it should only be done for track/event cars