How serious was the loss of Massey’s Hook for the Stormlands after the Conquest? How about the lands near the headwaters of the Mander prior to that?

I’ll discuss this in detail when I write my Politics of the Stormlands essay, but the Stormlands we know today is something of a mutilated kingdom, less than half of what it used to be historically (even ignoring its Riverlands empire for the moment).