This is a question that needs more context. Are we discussing a geometric X-max and an appropriate damper surround design to allow that excursion? Because many loudspeakers do not have the full mechanical capability that the manufacturer states. Klippel can be gamed. I once did verification measurements on a subwoofer that claimed 12mm greater excursion than was mechanically verified. All measured via Klippel. All measurements are subject to the honesty of the person doing them.I haven't done the math, but what's the worst case change in SD at full xmax?

So what happens as you approach the mechanical limits of a surround? It depends on the surround profile. Half roll you can actually engage a little more than half of the roll.