X58 chipset based motherboards do not have native SATA 3 (6Gb/s) ports.
Your motherboard uses a Marvell controller for its SATA 6Gb/s ports. The controller has a bandwidth of PCIe x1 at 5Gb/s (500MB/s).
You won't get advertised Read/Write speeds from any model SATA 6Gb/s SSD you connect to your motherboard.
SATA 3 (6Gb/s) speeds are from 301MB/s to 600MB/s. It's possible that your SSD is connected to a SATA 2 (3Gb/s) port.
Double-check that your SSD is connected to port SATA_6G_1 or SATA_6G_2. Page 2-2 of your motherboard manual will show you the location of the ports.