The one detail which I believe makes obvious the fact that Julian was retroactively written as a genetically enhanced character is his cockiness early on in the series. Knowing the character as we came to, I don't think he'd have been quite so snarky at the gitgo had he been hiding such a thing.
I've heard this point before, but I don't really get it. What's the difference if his genetic gifts are natural or artificial? They're still genetic gifts either way. He still put in the work and years to be a doctor.