An update to my prior post's numbers on memory consumption: it seems a large chunk of the apparent increase in GNOME Shell's memory usage in Fedora 20 is, somehow, related to the fact that I tested in a VM. Testing on metal does not reproduce the same numbers: I see 173MB for Shell in top, only slightly higher than F19's 160MB in a VM. I haven't yet tried F19 on metal, but I will.
So far the theory is this is due to llvmpipe-based software rendering of the Shell being used in the VM. F19 was already using llvmpipe, so this would mean there must have been some kind of change in its memory use or memory use accounting between F19 and F20, but the situation does seem to be more complicated than just 'Shell uses a ton more memory in F20 than F19'.
Many thanks to Jasper St. Pierre and Matthias Clasen for guiding me through a closer investigation of this, and our work has already borne some fruit: I also did some valgrind profiling, and that led to Jasper finding some issues with desktop background loading that should save 12MB or so in my test configuration.
I'll keep on poking at this if I can find time, but right now work on F20 is taking priority. Unfortunately, it's looking a bit like we may have to slip the Final release by a week, but we'll make the final decision on Thursday...