If you check out these so called 'mods'(I normally wouldn't even call it that, small changes in physics would be the right declaration) thoroughly, you'll almost always see that the cars he uses as basis for his change of physics are looking quite different than the GTL car where he puts it in.
For example the V8 Capri
http://upload.worldofplayers.de/files3/34038.jpg
http://upload.worldofplayers.de/files3/330701.jpg
Does this look like the GTL Capri? No not at all. Different wheels/rims, totally different bodywork.
Same story with the Porsche 934, 911 Turbo street using rims from GTR4u without knowing who made them and so on.
Or the other 'tuned' stuff, like the 'Tuned Porsche' which should represent the Yellow Bird of RUF, they didn't even take the effort to put on the correct wheel/rim combination. Not to mention all the Mini stuff. Mini powered by Honda

Cars like this are better off in a game like NFS or Flatout or GTA SA.
If you want to see how a FULL MOD is done check out GTR4u Mod section, there are people who take this story very serious. There are cars in work like the brilliant Skyline GT-R, BMW CSL Turbo with changed bodywork and more. Another good adress is Butch:
http://www.kcendra.com/nfsbolide/nfscars.php?
game=gtl
He gave us the lovely Aston Martin DB4.