I'd like to pick up ARMI's Cairo, but it only says it is compatible with FSX. Does it work in P3D as well? 

I cannot remember which one I bought ARMI or SDG, but it worked.  Granted I also had EMT in some shape or form as well.

Sorry, I have SDG ,they have released an update for P3D v3 (works in V2.5 as well).