Genghis,
You wrote:
I notice however that the AAIB didn't actually state (although it's easy enough to work out) that using the CORRECT weight, and the correct safety factors, the runway was about 240m too short. (Whilst still being 3.3" out of aft CG!).
I presume this would involve extrapolation beyond the limits of the performance information in the manual, which some manufacturers get very upset about, and which may indeed be invalid.
A purist might argue that the CofG being out of limit would also invalidate use of the data, whether within or without the graphs or tables.
Finally, there might be the effect on a reader. Faced with an accident report which included reference to performance calculations for a weight over MTOW, he might believe there was something justifiable in flying overweight as long as the data was extrapolated to show that the runway was long enough...