With the knowledge that four SATA PM enclosures can sometimes perform better when limited to four hard drives per enclosure, that setup was also tested with two Tempo E4P host adapters. As you can see in the table below the 16 drive setup that utilized dual Sonnet E4P cards was able to provide 880MB/sec. READ performance.
Adding a 5th SATA PM enclosure with the other four drives for a total of twenty hard drives on two E4P cards was able to provide 1060MB/sec. READ and 870MB/sec WRITE performance.