With the disclaimer that I know absolutely nothing about Boeings - I'm a little aeroplane expert, the effects of being out of aft...
- Overly light pitch controls. In a manually flown aeroplane, this increases pilot workload, in a computer flown aeroplane, this may muck up the control laws and cause a loss of control.
- Increased risk of a neutral or divergent SPO.
- Increased risk of a PIO due to probably more rapid pitch response.
- Inability to maintain airspeed (anybody who has flown a CH601 will be very familiar with this).
- If manually flown, increased pilot workload to maintain speed and height, which can cause reduced effort elsewhere for other tasks.
- I doubt you'd get a take-off tailstrike with a manually flown aeroplane because the pilots self-protection mechanism should stop him over-rotating beyond what he's used to seeing out the window. An AFCS may not be quite so clever. Of course- with a very aft CG the pilot may not have the chance if it over-controls too much, but I think that the inertia of a big jet should prevent this, which I'd consider a little aeroplane problem. A landing tailstrike is more likely because you tend towards full elevator in the flare which you don't tend to in take-off.
And anybody's sim experience will almost certainly be useless in terms of what really happens out of limits. Sims are produced using safe middle-of-the-envelope FT data, and virtually never with out of envelope data. The computer will try and extrapolate, but the odds are it'll get things wrong.
Incidentally, when we certify light aircraft we invariably (not written down anywhere, just good practice) put some fat on the CG limits - typically ½" at aft minimum. I don't think this practice is so common in the big aeroplane world due to a combination of a commercial need to make aircraft as flexible as possible, and the assumption that controls on how the aircraft are loaded and CG calculated are much tighter than in your average part 23 aeroplane.
G