Personally I found the -wall mounted APs to be less powerful when compared to the standard EAPs
The wall versions have different chipsets and antenna setups based on their much smaller form factor, they are literally crammed into a wall. Cant say with 100% certainty but they do appear to be a lower spec hardware in the wall devices, think this is by design
Also bear in mind the designed market for -wall devices, they advertise them as dormitory / hotel room WiFi APs, both these scenarios wouldnt have a massive number of clients and certainly not the 40-60 you would be running
If I was specing your setup then I would be looking at the EAP245 or EAP660 which would be a closer replacement to what you already have.
In terms of comparision to the MR18, that is a N grade device if memory services me correctly?... RSSI Signal wise it should be roughly the same, im my experience this doesnt change much between vendors.. ultimately signal is signal, you environment is what effects this most and not the AP par say. However its my feeling that adding in wall APs vs Ceiling mounted would completely change the operating environment you are running in, therefore a new heatmap / RSSI review would be required. You would be much safer to just replace 'like for like' with ceiling mounted in the same position as the old ones, moving these could affect performance in one way or onther.
That aside, in terms of performance the addition of AC / AX speeds would certainly increase the performance for the user vs N grade.
Sell it to him by connecting an AC device and doing speedtests, lot of customers like to see the 866mbps connection speed vs older 300mbps