I don't think this is such an important question,it seems to me that for the Golden Company to take the Stormlands won't be a problem,since there are houses such as Dondarrion that were never too fond of the Lannister kings.The main question is the takeover of Storm's End hurting Stannis' pride,since now he lost the two castles he could call home (Dragonstone AND Storm's End),and as we saw with Robb,a king who loses their home isn't a victorious king.