Only if you have the 2006-2008 Mac Pro - you can disconnect the SATA cable where it connects to the controller near the front of the motherboard, and connect a PCIe SAS / SATA controller.
I believe (if you hunt on these forums) many people had success with Inateck controller cards, particularly if you needed the drives to be bootable. But, with this approach, you'll need to create an extension cable to connect the card to the motherboard connector, to take over the 4 bays. You may also need the PCIe power cable (connects to motherboard; usually used for graphics cards) to give it more power than it can draw from the PCIe slot normally.
On the 2009-2012 Mac Pros, this is impossible as the motherboard connection / controller is totally different.
What most people suggest there, if you *must* do it internally, is to use a mounting solution for the optical drive bays (you can mount up to 4 2.5" SSDs there with certain ones, I believe), and run SATA cables down through the machine to a controller card. This is really difficult because the space is *very* tight, there's not really any space designed to run cables easily, and it's also easy to accidentally detach a cable from a drive when installing the optical bay sled back into the Mac.
I would suggest two much simpler solutions.
- Mount your drives in an external SAS RAID enclosure. There are plenty of those on the market that are well-supported with Macs.
- Switch from SATA SSDs to fast PCIe SSDs. The Amfeltec card can mount up to 4 M.2 PCIe SSDs which gives you a stunning 5900Mb/+ throughput. Barefeats reviewed it here: http://barefeats.com/hard210.html
The latter is not a cheap solution, but it is the absolute fastest thing you can buy that goes inside a PCIe slot for now.
thanks