How to troubleshoot 802.11ac bandwidth problems

I would run /System/Library/CoreServices/Wireless Diagnostics to see if the tool can pick up on any problems with channel or interference. The drop off in speed seems too large to account for a switch from wired ethernet MTU to the overhead for WiFi, but it could be something that will get tuned with an update.

The good news is you get free support within 90 days of purchase so you could call AppleCare or visit a genius bar to review the Wireless Diagnostic scans and have them escalate the case to engineering at no cost to you if this slowdown persists. I'm sure they would be very interested in a test case where the hardware and software isn't working as expected.

In the mean time, you might have more details on SNR and Transmit Rate that might point to a reason why your transfers are slower over wireless than over wired.

You can even see how this tool can be used in action in this answer - where I turned on a microwave to show that there was some interference from that device.

AnandTech has done some troubleshooting on this issue and has found some problems with the TCP/IP stack in Mac OS X. Hopefully Apple can release a software patch soon to fix the issue.

http://anandtech.com/show/7085/the-2013-macbook-air-review-13inch/10