If anybody invites me to github (pau.duran, GMail acount), I'll push an implementation of Arc in SBCL... It loads arc.arc right now, and continuations are implemented. Also has a little test suite.
It could happen that my mistakes are useful to you... ;)
I ended up creating a public copy of my own git repo not to mess with Anarki...
As for your concerns, I think you shouldn't be too worried. Just check it out, there is ample room for improvement, especially in speed, which I understand is your primary objective. No network yet, no threads, continuations are implemented as chains of closures (I honestly don't know better than that)... it just loads plain arc.arc, and I haven't checked it thoroughly.
For me, this was a very nice opportunity to learn about the CPS transformation. Arc is perfect in this respect (only 4 special forms). I'm pretty sure that, as an exercise, it's full of bugs and misconceptions, because I love to learn by doing (and make lots of silly mistakes along the way)...
I'll be happy enough if it helps just a tiny bit...