I'm not really sure what the question is there either.
Do you mean that the stories are written with a single hero, and that the D&D group dynamic (thus forcing the D&D "story" to be from a group perspective) is interesting in contrast with that single hero?
If that's the case I guess I'd say in my mind that might actually take away from my interest in a Conan world. When I play in a setting based on a book I can't help but go into that game with the preconception that the game is going to be similar to what I've read. In this case I think a D&D party would feel rather shoehorned in and would affect my immersion. IH is less magic dependent, it has more of a dirty, lived in world feel to me, and I'd have less issue with a group of Iron Heroes getting together to plunder and pillage and rescue maidens.
For me it comes down to westerns. Though many western stories have just one or two "stars" using IH to me would be like the Dirty Dozen; recruiting a group of fighters that are (mostly) heroes in their own right to work together, and it fits well into the genre. Using D&D would be like taking the DragonLance crew and inserting them into the Wild West. It's not that it wouldn't work on some level, but on some levels (for me) it just wouldn't fit.
Also, I would expect that such a situation would be easier for a group. Seems like you're saying that an encounter balanced for a single player would be easier for four players (if you didn't adjust the encounter). And that's pretty much an "of course".
If I've misunderstood, please let me know. I find this an interesting question.
I mentioned Conan because it seems like if you're going to play in a world drawn from and around a character like that, it might (and this could be an interesting discussion as well) be a good test of your chosen system to see how well it can model that character. I think Conan could be modelled pretty darn well in IH, but I admit to not spending any cycles thinking about him and D20. Mostly because the item dependency in (untweaked) D20 would seem to preclude him being modelled.
