Even if you had an experienced racer drive LFS, or any simulator, they are not going to be able to definitively tell you that LFS "feels real". They could tell you the parts that don't feel real, and they might be able to give some pointers on how to make certain aspects better, but they are not going to be able to say, "Yes, this is exactly like driving a real race car". The simple reason is that there is just too much difference between being in a real car and driving a computer in an air conditioned room. Sensory input is severely diminished, smells are missing, no real vibration, sound, heat, fear... it's all lacking. There is nothing anybody can really do about that with today's technology.
So, a real race car driver would be a benefit to the devs, but they would not be the "end all" answer for making a realistic sim.
I have flown some of the best aircraft simulators in the world. And although they do a good job of modeling the behaviour of a particular aircraft, they still fall short of flying exactly the same. For example, one airplane that I flew had a nasty tendency to drop a wing when in a stall if you were not in perfect trim (25,000lb airplane going into a spin... not fun). The simulator never did a good job of modeling this. It stalled and mushed forward.
I have flown MS Flight Sim quite a bit, and although it is very, very good, I still could not tell you that it perfectly models the physics.