Well, if speed is your main requirement, and you have an actual app that you want to run faster, I would examine it and determine a) what subset of arc you use and b) what subset of arc you need. If it's most of arc, then rewriting it in scheme might be pretty hard.
Of course, first determine how much speed you actually need. And as aw said, rewriting in a faster language won't necessarily help you scale much anyway. Finding where you need the extra speed is definitely a prerequisite to figuring out the best way to get it.
Ultimately, we can't really tell you what to do, because we don't know nearly as much as you do about the needs you have for your application, the bottlenecks that are occurring, or how much time it would cost to re-implement your system on top of scheme directly.
Lisp is great for bottom up design, but whether you can get all the way up to your application before reaching arc first is not something that we can't really judge from here. Certainly some subset of arc can be built as mere macros over scheme. But only you can determine whether that subset is comfortable enough to use, and fast enough to solve your problems.
