I play Istvaan pretty carefully without betraying many hints of allegiance, so I'm pretty amazed that you picked up on that. And yes, "drug-obliterated human wreck that barely remembers where he's from" plays into it heavily as well.
And yeah, at the core, he's still Caldari, though he doesn't consciously consider himself as such anymore. Subconsciously, he derives his hatred of Gallenteans, his homophobia, and his racism toward Minmatar from that element of his past. He definitely was a Caldari patriot once, but only residual elements of that remain. He's seen too much out in space to have much loyalty left to a single dirtball, and would probably only take notice if that dirtball and everything he once was, suddenly ceased to exist (foreshadowing). He's one of the few capsuleers around with direct evidence of the big, scary shadow play behind Sarum and Heth and The Other, and literally knows the universe is about to up-end itself in a truly spectacular manner - in a way, his scope has expanded beyond nations, so Caldari isn't the first thing he thinks of when he looks to identify himself. One shouldn't forget that he's a complete and utter sociopath, and will always identify himself as himself first, with family/friends coming second, people whose power he desires third, and everyone else being largely inconsequential. He's been a megacorporate CEO, then he became a capsuleer, then he became the godfather of a criminal organization the whole goddamn charted universe still fears... each step moving him further away from what he was, but each step of what he was still shaping what he is.
Another aspect of the character is his loyalty to his own, which I see as inspired by Caldari loyalty, whether to home, state, parent corporation, whatever. So perhaps not a complete 100% sociopath. There's that one value. He may be a drug addled thief who doesn't bat an eye at killing thousands, but damned if he doesn't have one redeeming value. If you fuck with someone he considers important to him, he'll go Niko Bellic on you.
Ooh, I got to yammer on about my character. I am pleased.