I understand that it should be slower with the double layer of conversion, but how much slower have you found it to be in practice?
Could you give us some comparisons? It would be great if we could use the latest version of scheme, even if it comes at a modest performance penalty. Arc isn't exactly designed with performance in mind as it is, so as long as it doesn't become unusable this could be quite helpful.
Right now getting to the "arc>" prompt is 6.2 times slower, but that could be from not loading the Arc compiler as a module yet or from using the r5rs library.