Hi Jordhy.
Normally, performance files can be found in the GP4/gpxgata folder or the GP4/gpxdata/performance folder, but if the mod was installed with CSM they might be the GP4/GPSaves/Carsets folder instead.
If you can't find them, try searching for this text string in the GP4 folder ( and include subfolders):
** gp3edit - performance file **
That should help you to find where the files are. (Note that "gp3" is not a mistake - gp4 files use the same first line in the performance files).