Tried it. It works. No problem. It makes a big difference when trying to find thngs in UserExtraContent.
I managed to replace 943 assorted files with 11 nice little PK4's. I could have thrown them all into one, but kept the different car types separate for clarity (ie: coach.PK4, Ddine.PK4, etc). Now I can find stuff. I'm happy.
Note for Hawk: WP & P did mention he was intending to pack these neatly but apparently never got around to it. Since I've just done it and tested it without any problems showing up, if you want the PK4's for the archives just let me know.
ETA: Hey I'll just throw them into the upload page and you can use them if you want to.
Done. File name is
Tidied_up_PK4s_for_WP_and_P_custom_cars.7z