Yikes, it was definitely not designed to do that. You should really consider enterprise grade hardware.
The only time I have ever maxed it out was when we had a 100 Mbps connection coming from the ISP. Sadly, we got downgrade to a 30 Mbps connection now.
My reasoning behind this was logical actually. I ended up buying a Cisco router (small/medium business class) which I do not recall the model number. I was then going to run the AirPort Expresses (2nd Gen.) in Bridge Mode. I figured, why not try putting the Extreme as the centralized system. Sure enough it worked and has never locked up like the Cisco did. I did get the Extreme replaced once by Apple for a failing antenna.
That says something about the standards that Apple makes its networking hardware to. The fact that it runs great on a 5th gen. Extreme shows me how much faster the 6th gen. would be once we either increase the load or get a better bandwidth package.
In the meantime, just to give advice, it really depends on the configuration. In my situation it was for a school so we have one network running for teachers and faculty and a BYOD network for students with just Internet access. It works beautifully and beat Cisco, Ruckus, and Aerohive out by a large margin on cost. Since this was my Eagle Scout project I wanted to make it affordable!
The only other option I was considering was Ubiquiti's UniFi. The problem I saw however was the fact that I could use the configuration of the server at the school. Changing the configuration and the way the network was "hooked up" was not an option, but the AirPort Extreme worked just fine when I tested it out. If we ever move to more bandwidth or more clients I will either replace the 5th gen. Extreme with a 6th and still have a centralized system, or set all of them to Bridge Mode and put a more powerful router up front. Either way, I have no complaints and we have pushed some Expresses to 45 clients on special occasions at the school.