Feral ghouls are crazy people... They're not zombies... So no, I'd not like one of them to chase after me if a blow a limb off.
They should let out a whine and then huddle back into the darkness.
I think you're right about that, ghouls aren't zombies, they're people who have been changed by exposure to large amounts of radiation, while in some aspects, their physiology may have changed beyond the obvious features such as looking like rotting corpses (After all, you can't explain having an abnormally long lifespan as being the result of a few radiation burns.) I think a severed limb would still be kind of a problem, so I'd say that dismemberment should work the same as for humans.
I honestly don't think Obsidian took as much inspiration from "mods" as they are claiming. Almost every "new" feature they have added to New Vegas, Bethesda removed from the original games. Saying "we took inspiration from the mod community" sounds a lot nicer than "Yeah, we fixed all that [censored] our publisher cut when they inherited the franchise".
I don't recall hardcoe mode ever being in earlier games, or needing to eat and drink by default, and that seems to be Obsidian's biggest selling point as far as new features go. And in any case, I get the impression that Obsidian's decision to add new features isn't so much a matter of "bringing the series back to its roots" as trying to make New Vegas closer to their vision of what Fallout "should" be, which can be different things, after all, Arena was pretty much as generic as fantasy RPGs come in terms of setting with very little unique lore of its own, yet I doubt many Elder Scrolls fans would want Bethesda to retcon all the lore they've created for the series over the years and go back to being like Arena.