Wow Andrew, I never knew it was so complicated, Changing the GDB File had always seemed to work for me, I never use reidiculous amounts of cars anyway (Nothing over 40)
I must give that AIW edit a shot!
EDIT: Most Add-on Tracks AIW file is already open and ready to change. Sometimes I change the Race Pace and Qually Pace settings and they work a treat!
EDIT2: But anyone reading this, dont edit your AIW file without knwoing what you are doing, It could seriosuly mess up your game.