Yep, it appears to be the case. It runs faster without the prefetch instructions.
The doc (AMD64 Opt Manual, page 105) has to say do this:
"In some cases, using prefetch instructions on processors with hardware prefetching may slightly reduce performance. In these cases, it may be necessary to remove the prefetch instructions. All current AMD Athlon 64 and AMD Opteron processors have hardware prefetching mechanisms."
Another mystery explained.
Thanks