Yes, the mSATA slot is SATA 3 protocol. There seems to be a lot of confusion regarding this port as it is not clearly written in any of the support manuals. Here is why it is not clear. There are 2 SATA 3 protocol ports and 3 SATA 2 protocol ports. Looking at the conventional SATA data ports you only count 4 ports. The manual clearly states one of the SATA ports has two logical numbers that are 1 AND ~ OR 5. Since the Intel controller by default will support only two SATA 3 protocol ports, those ports on this motherboard are port 0 and mSATA.
If you are not confused yet, read on.
The blue SATA 0 data port on the XPS 8500 motherboard is SATA 3 protocol compliant. The white color port SATA port number 1 is a switching port. If the mSATA port is not used, it will become SATA 3 protocol compliant. If the mSATA port is used, the white port becomes SATA 2 protocol compliant and becomes SATA port number 5, so therefore it should be the last port used in your configuration.
Examples:
One SATA 3 protocol compliant hard disk or SSD installed... Use the blue port for 6Gb/s
Two SATA 3 protocol, one HDD and one SSD, use the blue port and white port for SATA 3 protocol 6Gb/s
One SATA 3 protocol mSATA and one HDD or SSD SATA 3 protocol drives, use the blue port for the second SATA 3 protocol drive. (The white data port 1 now becomes SATA 2 protocol compliant and changes port number from 1 to 5. ) This happens because both SATA 3 protocol can serve only two ports.
I hope this clears up some of the confusion about SATA 3 protocol, and port numbering ubiquity.