  Replacing with SAS will probably help, but your bottleneck is your controller, not your disk type.  There is no easy way to migrate a Windows installation from a SAS 6/iR to a PERC 6/i, but if you were to move to the PERC, you would see a substantial improvement.  The other issue is that since you can't mix SAS and SATA in a Virtual Disk, you would have to do a backup/restore to the SAS VD (you wouldn't be able to rebuild a SAS disk into the existing array to replace them one at a time).   Yes, you can add SAS drives to the system in addition to the SATA drives.  However, note that 1) the SAS 6/iR only supports 2 Virtual Disks, so only one VD can be added, and 2) the SAS 6/iR only supports RAID 0 and 1, so the only way to add "3-6" disks would be in a RAID 0, which would leave your data at extreme risk.    
Hello, We have a PE510 server that came with 2x 7.2k 2TB SATA drives and a PERC SAS 6/iR controller.   The machine servers as both a SQL server (its original purpose) and later on, a pretty busy file server.  The file server role was not originally intended for this machine.  What we are seeing is very bad I/O bottleneck during daily operation, even though only about a dozen people access this server.   (I have had the system checked out by support on more than one occasion). I'm wondering if replacing the drives with SAS will help resolve this.  There are 6 empty bays on the system. Would I be able to add 3-6 SAS drives to this system (in addition to the SATA disks installed), in order to create a new RAID volume and move files to it from the SATA volumes?   The existing SATA drives are in a RAID1 volume. Thank You   Thank You
PE-R510 adding SAS drives to system with SATA installed