How much an influence on flight/handling characteristics does it have when stab trim is incorrectly set for takeoff (e.g. 5.0 instead of 7.2) on the real plane?

We had quite a few MD-11 pilots in on the beta test, so I assume the simulation is fairly representative but it is impractical to simulate control forces and this is where you are going to feel the difference. The result might be a slightly longer takeoff roll but remember the trim is there primarily to neutralize control forces.

On the larger types, STAB TRIM really there to allow the tail to operate in the optimum configuration for that mode of flight. For lower speeds, higher trim values are necessary, because the elevator is simply not powerful enough at 1 ANU (Aircraft Nose Up) to rotate the aircraft at Vr. At cruise, you'll note the TRIM is set to a lower value (about 1 ANU is what I saw when I glanced the other day).While in a Cessna, or similar smaller type, trim is there to decrease the force necessary to hold a particular pitch angle, in an MD-11 or Boeing 747, it's there to adjust the entire H.Stab to make the elevator more effective. If an elevator can move +5 and -2 degrees in its range alone, placing the leading edge of the H.Stab at 5 degrees down (which will result in a pitch up) places the elevator's effective range at +10 to +3 degrees. This concept is not mirrored in many smaller aircraft, as their trim tabs are just holding their elevator at a certain pitch using aerodynamic force within the elevator's own pitch range, whereas on larger aircraft, this positioning is mechanized through jackscrews, and the elevator still has its own range outside of the motion of the horizontal stabilizer.If you head over to Airliners.net and select Tail/Wing/Winglet views and search, you will get a number of photos showing aircraft tails. If you look at the leading edge of the H.Stab, you will often see black lines showing the maximum pitch settings for the H.Stab for the aircraft.To answer your question, using a STAB TRIM setting lower than what the FMC says will result in a larger pitch motion on the yoke to achieve the correct rotation on takeoff. Setting STAB TRIM to neutral, I can nearly guarantee that you will not be able to rotate the aircraft at Vr, and probably not even get off the ground until after V2.