Ropes sound like a great solution for various text editing problems in Squeak. I would love to see them working.
It's the sort of idea that makes you think "I've thought of something like that before, except for *that* bit... cool".
About the performance tuning - another approach would be to do an actual performance test at runtime (first time a string is created after the image is opened...) to determine the relative costs of these operations in the particular configuration.
Daniel