The Mourningdale fort bonus can not help us exceed 600 GS rolls, it can only cover a missing bonus adding to the max GS. Since it does not increase the min GS, and all bonuses adding to the max also add to the min, we will be forced to have a lower min GS, leading to less optimal roll chances. The one situation this may make sense is to roll with one T4 material, instead of all T5, allowing you to get 590-600 rolls with lower material cost.