All in all, LFS models non-tarmac surfaces as rigid planes with significantly lower grip than tarmac. So whenever you get yourself on the gravel/mud/grass, you don't get those things. Instead, you get what you'll get from catching your wheels on some thick and stiff ice.
As for the BL bumps I'm talking about, to REALLy hotlap, you DO need to take those "bumps" (more like concrete curbs) at pretty high speeds. If you check the WR replay, you'll see noticable suspension damage if you turn on F10.
Since memorizing the surface condition of an entire rallycross circuit gets memory intensive real quick, I suggest modelling some finite amount of COMPLIANCE onto the mud/gravel/grass surface. Though the surface won't change with each drivethrough, at least the ground would display compliance that RL off-road surfaces do for a given surface contour. ATM, the rallycross non-tarmac sections drives more like slippery concrete than actual off-road surfaces.