Wireless has this rather strange operation.. and it varies a lot from device to device.. as each manufacturer decides what the balance of download and upload speeds will be.
Apple has chosen the download as much higher priority over upload.. this is usually the case. Most ISP do not offer synchronous speeds.. with download usually 10x upload or more.. the new phenomenon of fibre with upload of very high speed has resulted in loads of people finding they cannot achieve the same upload.. however is the issue the TC or the wireless driver in the computer.
Please do a test benny.. you will need another computer with a very fast hard disk. SSD
Or you can use the built in network speed test.. iperf (just google for the commands and how to use them). You can usually find some cheap app that is a nice graphical front end to iperf so you can do the tests easily.
Then setup to do network tests across your lan.. from the wireless computer to another computer plugged in by ethernet.
You can then get up and down speeds in your LAN setup.. it might surprise you but there are differences built in even to the card and driver.
It will also give you a good idea of the absolute max speed your computer can achieve over wireless.
If you look at standard tests of the AE gen6 (they didn't test the TC but the results will be near identical).
http://www.smallnetbuilder.com/wireless/wireless-reviews/32158-apple-airport-ext reme-80211ac-reviewed?start=3
The results are actually the opposite to what you found.. they clearly show upload and download near to the same speed.
Interestingly the WAN to LAN speed also shows major difference cf LAN to WAN. But the opposite to you.. again with upload way exceeding download.
http://www.smallnetbuilder.com/wireless/wireless-reviews/32158-apple-airport-ext reme-80211ac-reviewed?showall=&start=1
To me none of this is surprising.
One set of results under one set of testing conditions will never match another set exactly .. the whole thing is far to difficult and it is highly dependent on the cards used.
Even the smallnetbuilder tests which are about as good as any.. are PC environment and not IMHO directly applicable.. but show trends.. Macs always seem to be well tuned for their own all apple setups.. well up to Yosemite anyway.
