I tested it, and saw that HFS is using always something like 7MB.
Explorer is using also the same amount of memory, ~20MB.
So, it is NOT a memory leak by HFS. The extra memory is not used by HFS, but by Windows itself, for caching and other things. (don't ask me, ask microsoft)
It is a good thing (most times): a good operating system profit by the unused RAM. Because RAM is just a waste of money while it is unused.
In beta23, the only thing i see that may have caused this behaviour is a new way of sending data that, on fast networks, improves speed greatly.
I don't think it would be a great idea to reduce transfer speed to gain "unused ram".