It could potentially harm them thought. The GT series is largely dependent on licensing to have permission to use cars and brands within the game. Many licenses come with great restrictions. Car manufacturers don't want to see banged up models of their cars, ergo no real crash graphics. Ferrari only allows F1 cars to race F1 cars, etc...
This is where the problem starts with modified game data. Some license distributors might not be happy about people messing around with their intellectual property and might thus deny PD further use of the license in future titles. What might seem like a trivial offense by changing game data a bit with no harm intended might have a great negative outcome for the gaming community in the future. Under this aspect I do believe that PD is not putting enough effort into preventing game data modification.
New rumors/notices that some modifications allow free access to payed download content is also breaching ToS. Since this is accessed the same way car stats are modifies, action needs to be taken.
I fully understand the reason why people like to mod their cars. GT5 has its short-comings, and people wish for them to be resolved. For above named reasons I think that taking matters in your own hands and illegally mod a game is the wrong approach. The right way to resolve this is to create your own game and try to overcome the short-comings of this series, it might be the next best hit. Changing something in a game through unauthorized measures can lead to many unforeseen consequences, and the last thing I want to see is PD having to suffer them.
