So, the same approach was taken to open up the game as S2U, meaning the boot files were dummied out (other BFF files copied/renamed as BOOTFLOW, PHYSICSBOOTFLOW, and PHYSICSPERSISTENT), BFF files unpacked/renamed and placed into proper folders.
The big concern with applying this mod is that there is no guarantee that the tyre HDT.bin file names are correctly named. There is nothing in the coded HDT.bin files that gives a clue to a proper filename. As an example, one HDT.bin is named "0000a000.hdt" when PHYSICSBOOTFLOW is unpacked and opening with a hex editor gives no Ascii filename within the code. Opening the file named 0002e000.xml (which is tyreshd.xml) gives you a list of HDT filenames, but again no way to link with all of the unpacked alpha-numeric HDT filenames.
So, unless the modder has insider info as to how those unpacked HDT.bin files are to be named, he just guessed (to be able to release his mod). If the modder just took a guess at the file names, the original cars could be using the wrong unpacked HDT.bin file when the mod is applied. There are also some other physics related files which would require a guess as to the filename (some could be sorted out by decoding, while others cannot).
The next issue is that all of the physics files are now coded. So, wading through all of the code for the addresses to setup default mass, inertia, friction, spring, bump, rebound, suspension geometry, gearbox, engines, etc, becomes a real chore. Most modders will simply copy the physics files of the original cars and change little or nothing, which doesn't make for a quality addon car.
So, I recommend using caution when using this mod. Because I suspect that guesswork was involved in naming some of the physics files. Meaning, the physics of original cars can be impacted if the unpacked physics files are assigned the wrong file names.
On top of that, many modders fail to include all of the LOD models. They simply use the high poly model for all LOD levels, which is an FPS killer with a full field of similar cars.
To really have a quality car, tools would be needed to create/edit all physics files to make it less of a chore to setup the default physics of a car. Also, there are probably parameters added/removed in various physics files (compared to what existed in S2U).
Whenever SMS patches the game and revises either BOOTFLOW, PHYSICSBOOTFLOW, or PHYSICSPERSISTENT, one would have to unpack the BFF(s) and go through the process of renaming files and tossing them into the correct folders. So, it's probably best to wait until they stop patching the game to start modding it.
My 2 cents.