Those are some powerful words from someone who practically begged me for a hacked car.
I personally wouldn't do a single modification for it to be used to cheat, all I did, I created cars according to their official specifications, because specs for some cars in GT5 are plainly wrong.
I did create some cars for fun (Prius GT300, wheelie Camaro, Hayabusa Fiat 500...) but those never left my garage, and are pretty obvious to spot.....
On a different note, the aero settings can be increased beyond their in-game limit, but as Seth said, cannot be changed or they'll revert to original limit.
Also, just to warn you guys, grip can be increased, there is also a multiplier for this, but thankfully the pp system recognizes this, and the pp increases dramastically.
If anybody has any concern or questions, they are free to contact me, I'll be happy to provide assistance.