From what I understand there is no ground effect in LFS. I used to race Formula Ford Zetecs in the UK a while ago and you could get some downforce from the flat bottom and pitch in the setup.
However in LFS I see a lot of setups where the car is setup in squat (higher front) which I would never have done in real life, especially not as drasticly. This isn't realistic because it would kill downforce and introduce drag but I think is done to transfer more weight to the rear as LFS ignores this?
But your quite right, in a real car hitting the kerb would disturb underbody downforce.
With regard to flat shifting, I agree it's not realistic and I too would rather race realisticly, but unfortunately when looking for ultimate lap times people will use any tweak necessary. (Which I suppose IS realistic ironically!?).
I actually started playing LFS because I'm looking for a sim to practice with before deciding whether I'm still sharp enough to go racing for real again. Perhaps Formula Renault club racing in the UK. LFS certainly feels very real through it's force feedback and the buzz you get from improving lap times is great. The community seems very active and knowlegable too with some great comptetition.