I didn't know how to do this too so thanks. PD have really improved the A.I. I hope they can continue progressing GT5 in the future as good as they are doing it now.
You can't really change the AI aggression. It just changes the AI cars. Setting it to 10 means the AI get faster cars. They still suck at driving them so there still isn't any challenge to be had in arcade or practice modes racing the AI.
