The first thing that should be noted is that for most practical workloads of a single mechanical disk, even SATA1 which offers a transfer rate of 150 MB/s would not present a bottleneck. The higher transfer rates are mainly relevant for either cached I/O or high-performance SSDs. The first case is rare enough not to make a significant difference in overall performance. The latter case is only a concern if your SSD actually is performing better than the link can handle even over a long term (SSD performance typically degrades as the disk fills up). So unless you are building a corner-case setup, you would not notice a performance difference, no matter the link speed.
Now the technical part: there are two SAS standards - the older SAS 1 with a data rate of 3 Gbps and the newer SAS2 with a data rate of 6 Gbps per link. Both include SATA compatibility. SAS1 controllers would negotiate SATA2 speeds (300 MB/s) with connected SATA devices. Most newer SAS2 controllers will negotiate SATA3 speeds (600 MB/s) with capable devices. You should simply consult your controller's documentation / spec sheet to see what it would support.