We have done some internal performance testing with PERC 6 which included comparasions of RAID5 vs RAID6.
The key difference between RAID5 and RAID6 is that RAID5 can survive only a single disk failure, while RAID6 can survive a two disk failure. To accomplsih this RAID5 keeps one parity disk, while RAID6 has two. The additional parity calculation that RAID6 has to do is a performance penalty which makes its performance worse than RAID5.

The interesting thing is that the performance difference is bigger when there is a small number of disks in the RAID array. As the number of disks in the RAID group increase, the performance differnce between the two types decreases. This works out really well because it is with larger RAID disk configurations that you run a higher risk of multiple disk failures. Use RAID 5 with smaller RAID configurations and RAID 6 with larger ones to balance the performance and availability tradeoffs.

Thanks - Todd

The internal work that I based my initial response on has now been published in a Performance Analysis Report for the PERC6. It is on DellTechCenter here - http://www.delltechcenter.com/page/PERC+6+Performance+Anaylsis+Report

Thanks - Todd