So I've set it up as an access point and all is working OK. Was a bit disappointed to get the v1 hardware, not the v2 even though it came out last year, though not sure exactly of the differences. It was a good price though. I'm unclear what is the latest firmware, each TP-Link "national" site lists different versions even within the v1 category and there is US and EU versions.
There is no "Access Point" mode so I just gave it an address on the same LAN and disabled the DHCP server. This works fine the only problem is with no WAN the unit itself can't access the internet for purposes of network time or firmware update checking. I can't really speak as to the router functionality as I am just using it as a "dumb" access point.
For the WiFi I am currently using it in the Smart Connect mode where it gives 1 SSID for all 3 radios (2.4Ghz, 5Ghz 1, 5Ghz 2) and this seems to work fine. On my previous router I had separated the 2.4 and 5Ghz radios into different SSIDs and manually managed clients but if this Smart Connect actually works that's easier. I don't have many 5Ghz clients.
In terms of speed, here are some iperf tests from my Macbook Pro on WiFi via the C5400 to two machines on my LAN connected with wired gigabit. Test was done from 1 meter away from the C5400 while client showed a connection speed of 1300.
First to a QNAP server with 4x4TB drives in Raid-5:
[ ID] Interval Transfer Bandwidth
[ 4] 0.00-1.00 sec 79.7 MBytes 668 Mbits/sec
[ 4] 1.00-2.00 sec 80.9 MBytes 679 Mbits/sec
[ 4] 2.00-3.00 sec 83.3 MBytes 699 Mbits/sec
[ 4] 3.00-4.00 sec 84.4 MBytes 707 Mbits/sec
[ 4] 4.00-5.00 sec 84.8 MBytes 712 Mbits/sec
[ 4] 5.00-6.00 sec 84.7 MBytes 710 Mbits/sec
[ 4] 6.00-7.00 sec 84.4 MBytes 709 Mbits/sec
[ 4] 7.00-8.00 sec 84.7 MBytes 710 Mbits/sec
[ 4] 8.00-9.00 sec 85.7 MBytes 719 Mbits/sec
[ 4] 9.00-10.00 sec 85.7 MBytes 719 Mbits/sec
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval Transfer Bandwidth
[ 4] 0.00-10.00 sec 838 MBytes 703 Mbits/sec sender
[ 4] 0.00-10.00 sec 837 MBytes 702 Mbits/sec receiver
Next to an older other MacBook Pro with an internal SSD:
[ ID] Interval Transfer Bandwidth
[ 4] 0.00-1.00 sec 104 MBytes 873 Mbits/sec
[ 4] 1.00-2.00 sec 111 MBytes 929 Mbits/sec
[ 4] 2.00-3.00 sec 113 MBytes 945 Mbits/sec
[ 4] 3.00-4.00 sec 113 MBytes 948 Mbits/sec
[ 4] 4.00-5.00 sec 111 MBytes 930 Mbits/sec
[ 4] 5.00-6.00 sec 113 MBytes 952 Mbits/sec
[ 4] 6.00-7.00 sec 112 MBytes 944 Mbits/sec
[ 4] 7.00-8.00 sec 112 MBytes 940 Mbits/sec
[ 4] 8.00-9.00 sec 112 MBytes 938 Mbits/sec
[ 4] 9.00-10.00 sec 111 MBytes 934 Mbits/sec
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval Transfer Bandwidth
[ 4] 0.00-10.00 sec 1.09 GBytes 933 Mbits/sec sender
[ 4] 0.00-10.00 sec 1.09 GBytes 932 Mbits/sec receiver
I double-checked that last one with a file-copy of a 3GB zip of JPEG images and it did indeed transfer at just over 100MBytes/sec.
Overall pretty impressive.....