Voice Coil Magazine, has conducted many, many driver tests over the years with very good data.
I would get a free subscription and peruse them.
Also Audio Express https://audioxpress.com/page/Voice-Coil-Magazine
is the website for the magazine but not all driver tests are available on the website.
Here are some Klippel test made by ErinH for the shallow SB Acoustic SW.
I really don't fully understand them, only that the XMAX is not what the manufacturer said it is.
It is very normal for the Xmax to vary based on the testing methods/interpretations of the data.
Very few published Xmax ratings are comparable with standards or each other as there are many ways Xmax can be thought of.
Even some Purifi drivers and many other famous drivers don't meet the standard Erin uses.
While a standard would be nice, singling out SB acoustics is disingenuous as they are not alone, plus their drivers sound phenomenal for the price.
Many companies are now using Klippel systems in their designs and drivers and often the specs still vary.
That said Parts Express recent Epique drivers and several drivers from MISCO/Bold North Audio met very lofty specs in real life and were designed with Klippel equipment.
MISCO/Bold North by the way is associated with or actually is Warkwyn, the North American distributor for Klippel products.
MISCO customers gain access to measure linear and large signal parameters, a Klippel Near Field Scanner for 3D sound radiation, and more—all in house.
I'd expect some pretty great products from them in the coming years.