The correct answer to your questions is that a standard 2.5 inch, SATA 3 6Gb/s solid stated drive would be limited to SATA 3 performance which is about 550 MB/s read and 535 MB/s write. That is the upper practical limit for a standard 2.5 inch ssd even if it is plugged into one of the two SATA ports in a motherboard's SATA Express header/port.
To the best of my knowledge there were never any SATA Express ssd's manufactured for large scale consumer use. The SATA Express standard was a last ditch effort by the international SATA organization to try and compete with newly introduced M.2 and PCIe ssd's. It was a lost cause right from the start. The new M.2 and PCIe ssd's perform better than SATA Express ever could.
Your motherboard is equipped with a M.2, 3.0 x 4, ssd header/port. You would be better off purchasing an M.2 3.0 x 4 NVMe ssd like the Samsung 950 Pro, Toshiba XG3, or OCZ RD400.