Yeah, for practical purposes, at least in american stanleys - once you get past the really early soft irons and thin castings, and before you get to the post type-20 planes, everything in the middle kind of works about the same. Some of the later irons are definitely a little bit lower in carbon (like the square top type 20s), but I haven't tested much other than really early and really late. More out of curiosity to see if there is surplus carbon in the laminated irons so that they could be driven higher in hardness (there is surplus carbon, but they do something to manufacture them and constrain or grind them - they turn into potato chips if you try to reharden them), and then on the softer block plane irons and 750 chisel to see if stanley made them soft and it wasn't a matter of a poor alloy (that is the case - both are very good. I think the 750s might be something similar to O1 - they can be made to match anything boutique if they're rehardened).
Stick to something that looks a little grayish with smoother machining if you're concerned about later irons being a touch softer. Until they get really soft (like the round tops), you might prefer them in something like a jointer.
Smoothers are really the only thing where a harder iron can be gamed, and even then, I don't think harder irons provide any benefits in an all hand tool cycle - more like for a smoother that's following wood out of a planer.