UPDATE: Ok I have found out you don't have delete the game data every time it will just quickly install any files you've updated which is a lot easier.
I have been able to swap cars and some of the clothes from ps3 version of TBOGT and TLAD also swapped Nikos face texture for Johnny from TLad, most of the files in common.rpf can be modded and a few of the files in ps3.rpf can be to, i have modded hud.dat to change the text colour, some of the files can be swapped for versions for the pc like visualsettings.dat, cargrp.dat, vehicles.ide, popcycle.dat the weaponsinfo.xml can also be modded, effects like blood and explosions can be modded with pc versions.
The only files i have have come across that seem to stuff it up are the sound files for the cars that i swapped for modded pc versions the game loaded and played for about ten mins the locked up with a weird buzzing sound, american.gxt and the rain effects .xmls might be able to mod the ps3 versions of them but pc versions won't work.
I will keep testing and update when i find out if more files can be modded, also i believe the pc versions .exe might not be needed jut right click on common.rpf,ps3.rpf or cdimages in the models folder and open with SparkIV might work i'm not sure haven't tested.
HOPE YOU ENJOY 2HIGH4U