VR Zone reports Gigabyte P55A motherboards with USB 3.0 and SATA 6Gbps suffer from PCI Express bottleneck issues. To fix the issue motherboard makers need to add a PLX switch chip but that will add up to $25 to the cost of the motherboard. You can find more info over here.
Hardware.info has some information about the PCIe bottleneck issues on Gigabyte P55A motherboards supporting USB3 and SATA3. There are 3 scenarios you could face using that board. One, your graphics card will run at half the PCIe bandwidth (x8) if you use USB3 or SATA3. Two, if your graphics card run at full speed (x16) or if you are on Crossfire/SLI (x8,x8), USB3 and SATA3 will run at half speed. Third, if you are running 3 graphics cards like tri-Crossfire or SLI+PhysX, you can't use x1 slots or the eSATA at all.