Max range depends on ammunition used, and since all those modern 155mm howitzers have the main armament with same characteristics (155mm 52cal gun), when using the same ammunition the max range will be similar.
And don't be to excited about all those max ranges from the producers catalogs. It's not that hard to make artillery shell fly that far, but it's hard to actually hit something, especially with those rocket assisted shells. Unless you are using something like M982 Excalibur, you can only dream about hitting anything at those ranges. Thats why today in even the most advanced armies you probably won't find shells that are supposed to fly over 40km. And you probably won't have effective and precise fire over the range of 30 km, unless you are using some kind of expensive guided munition.