We had duscussion here, eons ago, with Old Smokey et al, (might have involved SSG, JT) on the subject of overruns. First, the risk analyst those classify an overrun as "catastrophic" at, IIRC, something like 70 knots. Second, very few extra knots at the initiation of the high speed reject adds lots of knots at overrun point. Something like 5 knots extra at the first initiation results in 40-60 knots at the runway end.