The setups your using are probably setup to be fast in LFS, rather than in the real world - if you perfectly replicated a real car in LFS with an exact setup chances are it'd be pretty close and a lot more understeer prone just like the real thing, but to be alien fast in LFS you need an alien setup. For example, the default setups that ship with LFS are slow, bouncy, understeery, but they seem pretty realistic to me.