So due to the stock firmware restricting country codes - I've installed OpenWRT on myEX3700. After a surprising bit of trouble to find a reference implementation, I've finally got something working with only one problem - it is SLOW.
First Hop (DL/UL) - 198.75/119.38
EX3700 w/ OpenWRT - 18.89 / 0.35
I've checked for channel contention (doesn't seem like it). Could this be the natural speed?
Configuration in question: Internet (WAN) <-> F660A router <-> EX 3700 <-> Clients DL (Unknown Mbps) <-> (~200 Mbps) <-> (18.89 Mbps) UL (Unknown Mbps) <-> (Unknown Mbps) <-> (.35-1 Mbps)
I am missing some results from testing directly on the device that show the upstream router connection on 2Ghz performing within expectations. Will try to post those later today.
Regarding a reference implementation - this could be me approaching the problem incorrectly or failing to google. I would be looking for a "WIFI Extender" Recipe that sort of makes the decisions for me on whether to use relayd or just NAT the connections. Perhaps also addressing if Wifi extenders running native firmware are passing packets via the kernel or some sort of hardware pipeline (i.e. skip the CPU of the ex3700), which may explain the performance degredation. I saw some references to this being the case on other hardware. An example is here: