I don't buy it.

Replacing Emacs elisp implementation is just looking for trouble. Although it is not the fastest one, the existing one works well enough and is battle tested and there are millions of lines of elisp code that would need to be tested, debugged and fixed.

If the goal is to allow to write parts of Emacs and extensions in some other language, I wander is Scheme really the best choice? Why not go all out and implement elisp in Common Lisp and use Common Lisp as a base for future Emacs and extensions development. There have already been such attempts, so it's not exactly something unheard of.