I'm not saying that if you would have gone into the the Sorting being smug about how gifted you were, that means you were a rich overprivileged racist dickhead. But you probably did have a certain level of personal history that meant you were confident your cleverness would be properly recognized and would be treated as an inherent virtue of you as an individual

I think recessional has gone over this ground already but nooooooooo that was absolutely not my experience as a Smart Kid (TM) in the US school system. I think you're confusing the Slytherin-like "People will recognize my inherent awesomeness and I will wind up in the place where I am rewarded for that" with the idea "If I get to a place where people value the same things I value maybe I won't be treated as a complete pariah and can be happy being myself." Those are two pretty diametrically opposed views of the self. I don't think the idea that "I will be Recognized and appreciated" is entirely privileged or fanon -- Harry himself is in the grand romantic tradition of the heroic orphaned child who isn't appreciated by anyone around him and is lifted up to greatness by how awesome he inherently is! It's a genre the book is actively participating in.

Muggleborns who ended up in other houses either because getting their Hogwarts letter knocked that sense of confidence in the world for a loop

But that's totally not the narrative. The narrative is "I get the letter and my world is changed and that's awesome."