What do you mean by underpeformed? It opened far bigger than Ratatouille, and while that can be attributed to having grand marketing, the silent factor didn't deter people from coming back, as it made nearly four times its opening weekend in total. And $534m worldwide, 4th highest Pixar gross. If the general public had the kind of reaction we might have expected from them when they actually saw the movie and were turned off by it, it would have fallen off, but it stayed strong and soared to smash success just like every other Pixar movie. You risk a lot with a bait and switch marketing campaign, mostly the film's box office legs and reputation (tempt the wrong audience into seeing a movie, or show them something it isn't and they'll be pissed and crap all over it for all time, which seems like what will happen with The Road) but this time they did OK.
Perhaps why Ratatouille stands out so much for me is how unexpected it was, the marketing wasn't good at all, the only reason I was going is because of my trust in Pixar, but then it shocked me by not only being up to par with their films, but having a beyond impressive storyline, that wasn't at all kid friendly, its definitely where they stepped away from making kids movies despite the marketing and general opinion that animated movies are for kids. Also the animation in Ratatouille is simply stunning (I'm not as enthused by the character design, and the one thing I've always somewhat disliked about their movies is their rendering of people) but the whole picture surpassed Finding Nemo for me in terms of sheer visual quality, Cars had some incredible vistas but not the ridiculous detail that FN had. Their movies have always been more interested in storytelling that anyone can appreciate, their early movies are like that too but I feel they're still easily digestable, but Ratatouille, Wall-E, and Up are stunning for their artistry and weaving in of thematic material.
That said, their first movies weren't just fun for me when I was a kid, but still stand up as awesome stories with excellent humor now, I'll love Toy Story for the rest of my life not just for nostalgia (as with some of my favorite movies as a kid) but because of how much of a classic it deservedly is.