It changes everything, 2016Generic.txt is the performance file (of cars, drivers, teams) and 2016Generic.g3p is the file for the physics (rpm, weight, downforce, brake balance, etc) so if you don´t have a g3p file you will have the original game values (more than the 15.000 rpm limit, less torque, etc.) so if you want realistic behaviour of the cars you must use a g3p file (in case you´re using CSM, otherwise you could edit the executable I guess) but ask to
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, he will explain it better and shorter