Slow GTX 780M in 2013 iMac

I just took delivery of a late 2013 iMac with a GTX 780M installed. I installed the latest CUDA release (5.5.28) and ran a few samples to check everything works. The device-device bandwidth seems very low (32GB/s) in the bandwidthTest sample. The bandwidth in the specs is 160GB/s (, I know that actual bandwidth is usually less than this but not less than 1/5th! Has anyone else had similar issues and been able to resolve them?