400 concurrent users registered under only one Wireless AP?
Apart from the theoretical 255 devices limit (to be verified), aren't there also hardware related limits regarding (a) the real users bandwidth usage ratio (2 Mbps per user, concurrently and continuously? or 2 Mbps in bursts per each user? if so which ratio? 1:10?) and (b) the maximum number of devices per SSID?
Not sure but...I personally think that so many concurrent users, no matter the throughput between that AP and the rest of the network it is connected to, communicating on a single AP is, first of all, a practical nonsense (could be a requirement but that doesn't mean it is a reasonable requirement, to me it is totally wrong)...and secondly the hardware supporting technology isn't limitless (see considerations above, just to start).
But...you already know the answer to your question (here), provided directly by Aruba experts.