15ft is not much of a margin even with the gross/net buffer.
Again, gross/net progressively improves whatever you have at runway head as the departure progresses.
So , I end up on the grass doing 4 knot, bu hu.
However, the reality of the dynamics of stopping is that the rate of speed decrease is quite high. The overrun is far more likely to commence at, say, 80-100 kt, than near stopped. On the runway, this speed is washed off quickly without damage (apart from very hot brakes) but, if you hit some rough things at speed, the damage may be significant