Thanks for the posts, I will try that tomorrow. I have all my FS9 backed up several times, so don't worry about that!
There is a programme called FS9 Configurator which edits that dll file (to make traffic move faster for instance) but I don't think it can do this. I'll double check, then try what you suggest (though, does it work in FS9? I hardly ever fly FSX to be honest and I don't even have P3D).
I used to run AISeparation and AISmooth every flight as a matter of course, but since I started using PF3 (rather than PFE) I have stopped running them. I can't say that I notice any difference to be honest.
I vary the % AI traffic according to airport - it's a trade off of course between empty gates and problems due to overcrowding. I usually fly with between 25% and 50% but I set 90-100% at small airports (or little traffic shows in the sim).