Your memory might actually be limiting your throughput. It appears you are transferring from RAMdisk to RAMdisk? What speed RAM are you using? Also you would be better off using diskspd instead of an explorer file transfer to measure performance.
Have you tried the nd_send_bw command line tool? I used that to verify connectivity and maximum throughput. I verified the numbers it was reporting by checking performance counters on my switch, which matched the reported throughput. I had a similar PCIe bandwidth limitation that I discovered by using nd_send_bw.