http://www.merriam-webster.com/dictionary/cheating
1: to deprive of something valuable by the use of deceit or fraud
2: to influence or lead by deceit, trick, or artifice
3: to elude or thwart by or as if by outwitting <cheat death>
#2 sure seems applicable to all this car modifying. So I would call it cheating.
I myself could care less what people do to their saved data for private use. What I do dislike is the possibility that these modifications leak into honest online competitions. Even worse is the suspicion that follows with it. It ruins the fun of those that want to compete honestly by placing them into doubt of having lost due to lack of speed, or because someone decided to manipulate the game files.
What everyone forgets is the greater consequence of all this modifying that PD could face. Some companies place great constraints on their licenses, for example Ferrari only allowing F1 cars to be race-able against F1 cars. Now I don't know the license requirements of each license PD has acquired, but actions like this may cause license distributors to deny PD future licenses because their property is being misused.
I don't understand why so many believe GT5 is dull without modding. There are so many possibilities available in GT5, there is no way even I in all my years of playing have had the chance to race all the cars in different environments. People demand DLCs, changes to game physics, and many other things. Why is it that this younger gamer generation is so spoiled on their game content...has everyone forgotten how to make the best out of what is delivered?