Didn't the Legions started using their Primarchs "current" gene-seed as soon as they had found them, though? So all the Marines we ever see in the 40k stories would have been created with the "mutated" gene-seed of their Primarch.
I do agree that the Emperor probably did not make the Primarchs identical. It's just that whatever he may have concocted in his lab, how the Primarchs turned out in the end was very different. For a lot of Primarchs, the influence of Chaos or the environmental factors are explicitly described in their origin story. It seems that now Gav has decided that the wolf traits of Russ had been "built in", and were not the result of Chaos influence. An odd decision, if you ask me. And not the only "odd decision" of the Horus Heresy series. (I don't even think I have o explain why I dislike it.)
I made mention that several of the key charactors in the Heresy (and feeding into the 40k universe) were created prior to them being untied with their Primarch. So in my opinion it weakens your point about the influence of chaos and strengthens the concepts in Deliverance Lost.
I've no problem with the view held in the Codex's and previous editions, in my opinion; it's how you view a legend and like all legends the subtlety (shades of grey, if you prefer) are lost. While the Horus Heresy novels have changed our view of the canon, I welcome the fact it adds charactor and sublety to the foundations of the 40k universe.