I recently built a gaming PC. After setting it up, I unplugged the Ethernet cable from my iMac and plugged it into my new PC. Instead of getting 1gbps speeds, I was getting 100mbps speeds. Since this was a symptom of mismatched wires inside the cable, I cut the head off and sure enough they were not in the proper order. I crimped a new head onto it with the wires in the correct order, and sure enough my new PC was getting 1gbps speeds. So Macs apparently have the ability to use Ethernet cables at their max bandwidth even if they are improperly wired.