Don't know if there's a "wow" score that is "wow" enough to get you a residency spot solely on your Step 1 score. They probably exist, but its very specialty/program dependent.
All you can realistically hope to gain with a Step 1 score is to not be automatically declined for an interview during the pruning process by being below their cutoff; or more specifically, having high enough of a score to prompt them to say "this is someone we'd like to meet face to face". Once you get that interview its all about how they see you fitting with the program and what they think of your potential competence as an intern. Can your Step 1 score help in biasing how they view your competence? Sure, but I dont know if you could put a number to that in the context of everything else on your application.