We played with sb-ext:bytes-consed-between-gcs, but couldn't find the right balance. That's why we were surprised with the result of the oversized heap experiment
You might want to do some googling about it, but I also seem to remember that SBCL might decide to use large or small MMU page granularity depending on the size of the heap. That might be the watershed trigger for performance. (or just some random mis-remembered nonsense)