Ian Difficult question. Since I don't have a set up to reproduce it, all I can do is to guess.
I don't think the number of wireless devices registered as connected but are not transmitting will reduce the number of new devices accepted. The reason is that this maximum number is not limited by software but a hardware limitation. It is the maximum number of devices with which RPi has enough power to actively communicate. (The relevant discussions from RPi have been pointed out in this forum.)
I assume that the number of stale devices are not very high. How many? Say, in a case where only 5 devices could connect, can you see 10 stale devices?