50 - 60 clients, depending on how much activity each client is doing, could be a little high.
What version of SW are you running? 4.0.0.3?
Is Client Match enabled?
5GHz will, generally, provide cleaner air than 2.4GHz; however, you coverage per AP is reduced. Have you done a site survey to see if you are getting interference from neighboring APs or other non-WiFi devices on 2.4? Not all devices support 5GHz so check the client types that are connecting.