Since this system only supports (SAS2: 6Gbs) and (SATA3: 6Gbs) and the Backplane only supports (SAS2/SATA3). So even you are trying to use SAS3 hard drives, the speed will scale down as 6Gbs.
Also, we have not tested/validated this system with SAS3 drives, so we do not know the compatibility issue might occur. You can try it out, but it is your own risks.