Byteguy wrote

Does this mean that if we install another AP (which we will need to do for better coverage) we will not be able to downlink to the root AP?

Yes. You can connect further APs to 1st hop mesh nodes rather than to their root node. Your current configuration already has one hop.

As for the performance, the number of hops isn't that critical since with more mesh nodes you won't be able to increase the capacity of the clients anyway. The whole mesh network will only serve as much clients as one single EAP can do. So, meshing can help to deploy further EAPs more easily (no cable connection needed), but it won't increase the capacity of clients.

If an AP can only handle 4 downlinks, how do you handle a really large mesh network?

By using more root nodes for the mesh network.

@Byteguy, when we discussed the network topology, I mentioned that a PtP link using CPEs limits the total bandwidth to 100 Mbps (80 to 90 Mbps in practice, see this story).

So, the answer depends on your Internet bandwidth:

• If you have more than 100 Mbps Internet bandwidth, you would need another PtP link (a pair of two CPEs) to increase overall bandwidth.
• If you have less than 100 Mbps Internet bandwidth, you could connect two EAPs to the same CPE using an outdoor Ethernet switch.

When using a PtMP topology (one CPE operating in AP mode and two CPEs operating in client mode) the max. bandwdith of the AP will be shared by both client CPEs.

IIRC we were also discussing to distribute the WLAN over a mesh link from the main building to the pier, right?

The point is that an EAP will give you 867 Mbps wireless throughput on the 5 GHz band and 450 Mbps wireless throughput on the 2.4 GHz band. A CPE link achieves only 300 Mbps in either the 5 GHz (CPE510) or the 2.4 GHz (CPE210) band. All values are WiFi throughputs over a half-duplex medium. The half-duplex goodput in 802.11n or ac mode is roughly wireless throughput - 30%, but the total goodput which can be achieved in practice strongly depends on usage (e.g. TCP vs. UDP protocol) and other environmental parameters such as interferences, AirTime etc.