I am concerned that you are new to CS and you want to 'improve' a working system's garbage collection code. But good luck anyway. Finding a new fast GC is like searching for the Holy Grail.
Also re the GC thing... the reason I am interested in is that Bill Schottstaedt, the author of s7, was not focusing on soft-realtime in the design of s7. His principal use was in Snd, his non-realtime Scheme based audio editor system. So while s7 is fast, there is (possibly) some low hanging fruit for making it perform better in the soft-realtime context of Max for Live, for example. It could well turn out to be either too hard or a dead end, but hey, that PhDs are for. :-)
Thanks! FWIW I'm not new to CS, I'm just coming from an interdisciplinary masters where the principal faculty side was music, while for the PhD those will be flipped. I am, however, new to compilers and interpreters... but I am talking about over the time span of a PhD so this is a multi-year worth of work question. I will be taking courses on compilers but would like to be studying the topic ahead of the course.