I think you're getting that 8192 figure from the controller datasheet yes?
Wired use is pretty different, and not so much of a worry in terms of this scaling/capacity figure.
I did almost exactly what you're looking at in pretty big campus about 3 years ago. In my case, there were old Cisco autonomous APs in the wired network (so they appear as wired clients to the M3).
Anyway, I think you should consider the max throughput of the controller firewall (20Gbps) and max active firewall session counts (524,000) first. Then blend that up with any "normal" AP work the M3 will be asked to do to work out where you're at.
My first thought, is that you might need to consider 10Gbit x 2 attachement of your M3 if you haven't already done so?