The M元50 G5 has 3 PCIe 1.1 x8 slots but electrically they are only x4 so that leaves us with 250MB/s per lane or 1000MB/s total bandwidth available to the HBA. The IBM M1015/LSI SAS2008 ( The preferred HBA for FreeNAS and ZFS) is a PCIe 2.0 x8 card meaning it can use 8 lanes of PCIe 2.0 at 500MB/s per lane or 4000MB/s total bandwidth.
Doing some reading i found that each version of PCIe has the following bandwidth accounting for overhead:
