The thing that is hardest to understand is the these Pro Drivers have such a powerful motor usually, that they are able to be pushed beyond their 'conventional' limits, according to the guys that are more experienced at experimenting with them. I don't know how else to describe it beyond paraphrasing one such designer: the Driver and the box form its own system, and when it's powered appropriately, you get performance that exceeds that conventional wisdom.
B&C, for example have stopped focusing on Xmax and use Xvar as their metric for safe excursion. I am still trying to understand this. You will also see many references about 70% of Bl defining excursion capabilities.
I've seen some home measurements by DIYers that have built some of these different cabinets using pro drivers, and their measurements tend to track well with the specs offered up by the designers. This would be true of the stuff from GSG as well.