@Plankton82  This is just in my experience. Moving between APs is the Roaming feature that exists with or without MESH.

In a mesh enviroment with two EAP225, if a device sits in the middle and the coverage is the same, to which of the two APs will it connect?

Roll of the dice has been my experience. Determined by standing in central location, disabling Wi-Fi on phone, enabling again and letting it auto connect.

Once it connects to one of the APs, if we move closer to the second AP, will it automatically connect to it although the signal to the first AP is still strong?

Likely not. I found that between APs, it would hold on to the first. I would actually have to move past the second AP quite a bit before it connected to the one with the stronger signal. You can play around with thresholds to force switching sooner but depending on your environment and other settings it could mess with static device connections.

If I wanted to connect 400 devices in an open plan area(I believe the EAP225 can hold between 60-80), would 5 AP be enough? Will the devices connect to the less congested AP or will they just connect to the first one they see? 

I don't know about the number of APs being sufficient or not.

Distribution of clients in my experience is shoddy. My network consists of 1 Omada Hardware Controller, 1 Omada Switch, 1 Root EAP610 OD and 4 EAP225 OD MESHED. There seems to be no reasoning behind how devices are distributed. Each of my EAPs is set for a max of six 2.4 and six 5ghz devices but it will max out one of the EAPs, seaminly ignoring another closer and stronger signal EAP.

The system does work and I have not had any major issues. I just feel like I have wasted EAPs that don't get utilized despite being the much stronger and closer EAP. I was attracted to TPlink because of the price and features, but now having this system for a while I find the features to be somewhat of a gimmick.  I actually bought equipement to connect my lot to the RV Park network. It cost more but is superior in performance. If I had the experience when we first built the TPlink network that I have now, I would probably have looked at other alternatives.

  @Plankton82 

Hey

Hopefully should be able to answer this for you!

In a mesh enviroment with two EAP225, if a device sits in the middle and the coverage is the same, to which of the two APs will it connect?

This depends on a number of things, there is no hard and fast answer.   Generally this will simply be the first AP to offer the SSID to the client.  However if AI roaming is enabled, the controller may move this to the other AP if the current one is more occupied (ease congestion).   The initial connection however, its totally random if the signals are exactly the same, however in reality that very very unlikely to happen. 

Once it connects to one of the APs, if we move closer to the second AP, will it automatically connect to it although the signal to the first AP is still strong?

This will be down to the controller to decide.  If the current AP has a decent enough signal and the new AP is better, but busier (serving more clients) it likely to leave the client on the existing AP until the signal degrades a bit more and the viability for a move is better.    The controller will always try to consider what delivers the best service for the client, signal isnt its only deciding factor. 

This is the difference between AI Roaming and standard roaming.    AI Roaming offers the ability to preemptively roam a client to offer a better service.  No point having AP1 serving 20 clients while AP2 does nothing, a 10 / 10 or 12 / 8 split would offer a much better experience overall, but may require devices to roam AP irrespective of the currently good signal and performance they are receiving on AP1

If I wanted to connect 400 devices in an open plan area(I believe the EAP225 can hold between 60-80), would 5 AP be enough? Will the devices connect to the less congested AP or will they just connect to the first one they see? 

OK a lot to consider there..  First thing that jumps at me is the number of clients you have connecting.   Yes the EAP225 can do 60-80 clients, but the airspace will congest LONG before you hit that hardware limit and this will be your issue.

Ideally all your clients should be on 5ghz and the APs on different channels, I would also keep the channel widths down to 40mhz unless you are going to use the DFS channels

In relation to the number of APs, personally I would be aiming for around 40 devices per AP if I was asked to spec this up, my recommendation is 10 APs.  Again this is depending on what you are doing, if its just idle you may be ok to drop to 8, however honestly I would be considering 10 APs.

10 APs

40mhz channels on 5ghz, 20mhz on 2.4

All seperate channels, the 40mhz will keep it isolated. 

WPA2 AES

5ghz roaming prefered, airtime fairness on 2.4