I would say 100 users would make the link unusable
... 30 should work fine on 951ui, you can do even 40, but I would not put more on a single radio if you expect good performance ... did you measure latency too or just speed? Try setting changing some options in advanced config section, play with HW protection mode (rts/cts recommended), lower the hw retries, try adaptive noise immunity, ... do a frequency scan to best channel for your setup ... signals between -50 and -70 seem OK, what were the average CCQ rates?
JF.