What is the RF utilization on the channels that the APs are on. It is possible that the RF is at capacity so users cannot transmit or receive any management traffic. Being able to set maximum clients to 255 is not something that is really meant to be done. While the CPU might be able to push traffic, the RF utilization might be too great.
There are too many variables to even understand what is going on. If you want to do real testing, only put that SSID on a single AP, so you can eliminate any effects of clientmatch.