I don´t have a real NAS, only some network shares and 4TB HDD connected to my router as "wannbe NAS" and I get something like 14 to 16 MB/s on my Mac G5 ... just tested with a 2GB file which took around 2 minutes to transfer.

What kind of NIC does the X5000 have ? Did you test the transfer speeds on AOS4 and/or Linux on the machine ?