I started digging after regularly feeling that two stage deceleration and wondering what it was all about. Digging into the AMM I discovered this info. I actually think it's very valuable information and as per the usual KISS principle the figures are designed to reflect the average line pilot. In this case, you would have to assume the two stage ramp up deceleration and I believe the QRH/EFB already factors this in. Ergo you can beat the figures using the correct technique on a dry runway and autobrake MED, idle reverse.
A real world example of this is CPH 22L. From the threshold to the intersection is about 1,100 meters. I would often ask PM if they think we can turn right onto the intersection to vacate and they would say no chance. And it is in fact, easily achievable. However the EFB would give numbers around the 1,600 meter mark. That's almost a 50% increase! Never mind a 15% margin! This example uses idle reverse also and at nominal weights, nil wind or less than 10 knot headwind. Also taking the corner at 15 knots too.
ATC always seemed slightly surprised when we asked to take 30 to vacate...