That model has native 6Gb/s ports (Intel), so you can get advertised speeds if your drive is connected to the Intel ports and you benchmark with ATTO.
I am with this post.
From the Asus site for the P8Z68-V PRO
Intel® Z68 chipset :
2 x SATA 6Gb/s port(s), gray <---Use this, but make sure you are in AHCI mode for best performance
4 x SATA 3Gb/s port(s), blue
Support Raid 0, 1, 5, 10
Support Intel® Smart Response Technology on 2nd generation Intel® Core™ processor family
Marvell® PCIe SATA 6Gb/s controller : *2 <---Dont use this for the SSD
2 x SATA 6Gb/s port(s), navy blue
JMicron® JMB362 controller : *2
1 x eSATA 3Gb/s port(s), red