XFX PSU

I have a XFX 550w full wired 80+ bronze. I recently bought the RX 580 Sapphire with a 8 and 6 pin power requirement. My PSU has two 6+2 pin ends that merge to one cable that works back to the PSU. I plugged it in and it booted up fine. I loaded up a game to see how it ran and it worked fine...