@SteveHeathfield 

I don't talk to the actual TPlink specfications, but I do have experience in real-world mesh optimizations.  I would never recommend more than 3hops+3neighbours as a rule, so that means a theorectical max 14 APs hanging off a single wired 15th AP, and only if the 14 can self organize into a reasonable binary tree.  If any kind of performance at all is needed, I'd really recommend using 2 x EAP225-outdoors wired up to the switch and operating the wired APs on different channels as that will be much more performant.

An example of a (barely) 'valid' mesh organization (in my opinion) as throughput will be terrible 

Starting from the blue Wired node #1:

  @SteveHeathfield 

Keen to hear how this progresses.

One other tactic you can use is in the Row2-Row3 links by joining the ethernet ports of the 225's (is this your 'reduced back-boan'?) with a cable (between their respective injectors), BUT fixing their 5Ghz radios on different channels.  The biggest issue with meshes is that all the backhauls have to operate on the same radio channel, which creates a lot of contention, and a lot of noise (as APs on the far right try to talk to a central node, they cannot 'hear' an AP on the far left and so don't back off and the L and R nodes wind up talking all over each other forcing a retransmit of all those packets, plus the lost time for the central AP to upload.

For every wired pair (tandem) in the middle of your mesh you can introduce a new frequency for use further downstream.  Again, performance tactic.  If you are clever about both selecting and fixing the 2.4 and 5.8 radio channels, you can mount the APs quite close to each other (ie same pole or whatever) to simplify the tandem wiring etc etc.