I just looked into the surface_materials and wrote that little tool to change the grip for all materials easily.
Unfortunately I had to notice that it doesn't save carreer progress with an edited surface_materials.xml file -.-
Maybe it's just my installation here, but that's very optimistic.
Maybe someone has an idea how to get around this!?
The tool makes a backup of the original file and can restore it with the button "Reset to default".
If there is any problem I will attach the original file here, too.
I'd like to tell you "Have fun", but it seems you won't have, unless you finished your carrer already and you are just doing races for fun
Original Surface file: https://dl.dropboxusercontent.com/u/36788053/surface_materials.zip
Tyre Grip Adjuster 1.0