Jesus christ,  
 
I just tried out Arcade mode with my "Stock" 458 Ferrari, on professional A class, I say stock because I haven't touched the engine/airflow/weight/exhaust instead of upping up the AI it just threw overpowered cars at me, I was racing Bugatti's and Citroen GT's  
 
I still thrashed them by 10 seconds on a 2 lap race of rome... 
 
Wtf game. 
 
EDIT: I haven't tried arcade mode since I bought the game when it was released...midnight purchase baby, so seeing how pitiful the AI really is was kind of shocking to me. 

I have never tried Arcade mode. I liked the Arcade mode in GT3 but after that one the mode has no value.

their is more problems with this game than i can shake a stick at.. I am still angry with gt5. i haven't gone back to it in awhile now.