gennaker, asymmetrical spinnaker

Hi,
I saw on some pictures of Mike Brooks CC87 that he is using an asymmetrical spinnaker.
Does anybody knows the dimensions of such a sail and whether this is more efficent then the genoa.
I saw on some pictures of Mike Brooks CC87 that he is using an asymmetrical spinnaker.
Does anybody knows the dimensions of such a sail and whether this is more efficent then the genoa.