Are you using RAID 0, RAID 1, or RAID 0+1?
oops....reread your entry. RAID 0, I see.
I would have to say that I agree with the earlier poster who suggests copying the same file to another internal drive. Also, as another poster mentioned, the compression that PSD files go through will slow performance. So, I would suggest copying files for your testing instead of saving from PS.
I am not a big fan of those benchmarking tests. They tell you nothing about the comparative speed of your hardware in real-world apps.
Is the eSATA device operating in SATA or SATA2 mode? Do you know?