"greatly improving performance" is an understatement! It was literally 3x. I didn't even have to change anything. Not bad for a language that usually beats python by quite a large margin :)
The Guile 2.0 branch. I don't know what magic optimisation dust they sprinkled over the upcoming 2.2, but it sure is fast.
We thought about using chicken, but it depends quite a lot on using syntax-case to deconstruct everything, and I didn't want to learn their implicit renaming stuff.
Apparently the 2.2 branch has full elisp support. Can't wait for Emacs to run on it.
We have a macro expander for pascal written in scheme (quick and dirty draft made by me that worked so well it stayed). We had some performance problems with some crazy recursive macros (it generates a _lot_ of code. Don't ask, I am not allowed to talk much about it), so I investigated porting it to chez.
Instead I just switched to guile (scheme implementation) trunk and got a 3x speedup. Did some optimizing work and it ended up at 4x, which is good enough.
This is a new favourite quote of mine.