An Xsan represents a lot of components working together seamlessly. Unless you have a ton of experience, you're going to get the best result by going by the book.
Treat your Nexsan units like they are Promise chassis:
http://support.apple.com/kb/HT1200
To get the result you want, it will be important to decide on a good block allocation size. You'll then use that to find your stripe breadth. Take a look at the link below and check out the section on file system configuration. If you need to move large files, start by experimenting with settings for Video.
http://www.xsanity.com/article.php/20060303050551499
JM