weberdarren97 :
I've found that even my R9 280X's gained performance when going from PCIe 2.0 to 3.0. There was almost a 10% increase in graphics.
For some reason, they also ran 3 degrees cooler on PCIe 3.0 than they did on 2.0... I wonder why... Same case and airflow. I even used the same CPU and memory. I simply switched from the Asus M5A99FX PRO R2.0 motherboard to the Asus Sabertooth 990FX/GEN3 R2.0 motherboard.
This has been tested extensively and I do not agree. Very likely there was some other issue with your old board either related to the power phases with your processor or something else to cause this change. There has never been a true instrumented test showing a 10% performance difference between 2.0 and 3.0, and its been tested with many cards, boards, and configurations.