Chris Wraights portrayal of them in Wrath of Iron is by far the best and so far only good interpretation. They do not only scream "Flesh is weak!" and hurr durr, but they also on a small atomic level act in every way machine-like. And they do things not just because they are stupid, but on a logical way it is the best alternative.
I also liked the small tidbits of Ferrus Manus who were worried that his Legion was flawed as they craved to better themselves (not unlike the Emperor's Children) through machines. The only fault in that is they are already perfect in every stance as possibly could be.
Imagine if Feat of Iron, being no stupid tale of rash Ferrus "I'm a hot head and hence forth stupid" Manus who is hunting aliens only to see the most obvious "You will be decapitated!"-prophecy, would build upon this. It could be prior to Isstvan V when Manus scribes his worries about the future of his Legion, how he and some senior officers would clash because he doesn't support their decisions to actively cut of their limbs, how he hate his own arms for what they are despite what they can do. And the ending would allude to them becoming fanatics rather than retelling the obvious ending of Fulgrim. In that way it would serve a purpose pre-Isstvan with tellings of the future. It would also show that they aren't now a days the most stupid Legion ever conceived.
That would add character, it would add conflicts and we would see them in a new light other than stereotypical re-takes of the overexagerated Codex.