If I want a Feral legendary memory who drops from a Layer 3 Thorgast for example, do I have to run it in Feral Spec?
Or can I run it in Bear spec? In that case, do I have to put my loot spec as Feral?
Memories ignore loot spec, you do not have to set your loot spec to feral to target a feral legendary.
You can run it as bear, or as resto, or as balance, or as feral, doesn’t matter. When you do content that can drop a memory, the memory will be on the loot table regardless of your current spec or loot spec. In Torghast, the droprate seems to be 100%, so a single run should get you the feral memory.