What bison160 was saying I guess, is you have to implement changes to both tire model and track's tdf file and figure out how they interact with each other to get the right feel. Yes, rF2 is not designed for that, neigther rF1 was, but it's always possible to get "close enough" workarounds. I bet Dirt Rally had even less source variables to play with.
Also, wouldn't hurt to increase track's mesh size (to 1x1m polys, or even 0,5x0,5 since the dirt ovals are small) and add some noise to the mesh itself. The physical noise is affecting the overall grip in a very interesting way, since the contact path of all wheels isn't flat anymore, and is different for each wheel.