Kim never had a perfect competition as a Senior before Olympics, but she did have many nearly perfect competitions. Kostner hasn't ever even had that. She always makes at least two big mistakes throughout a competition and usually makes at least three. I really don't see her being able to go completely perfect, but it's always possible. Her placement of that last Salchow in Bolero was overly ambitious; if she structures her jumps more like she did in 2012 then I could see her being able to deliver.
I think Kostner still would have been a couple points behind Yu-Na if she had skated clean at 2013 Worlds, though. She didn't have the same quality on the 3-3 combo and the program doesn't generate quite as much emotional impact as Yu-Na's. In general that's where Yu-Na has the advantage over everyone - her skating makes people emotional. I'm sure her SP to Send in the Clowns will be unbeatable if she skates it perfectly.
I think Kostner still would have been a couple points behind Yu-Na if she had skated clean at 2013 Worlds, though. She didn't have the same quality on the 3-3 combo and the program doesn't generate quite as much emotional impact as Yu-Na's. In general that's where Yu-Na has the advantage over everyone - her skating makes people emotional. I'm sure her SP to Send in the Clowns will be unbeatable if she skates it perfectly.