In 9.4dev, if the server is started with effective_cache_size = -1, then it cannot be changed away from that without a restart. If you change the config file and do a reload or pg_reload_conf(), it ignores the change without comment in the logs.
If you start the server with a value other than -1, then you can change the value by editing the file and doing a reload. You can even change it to -1, and then change it back away from -1 again.
It has been that way since at least 6b82f78ff95d7d4201d44359. Before that it was broken in other ways, so I don't know what the behavior would have been.
I don't know if bugs reports (without patches) against pre-release versions are supposed to go to hackers or to bugs. If someone has a strong preference, they might want to clarify this on the bug report form.
Cheers,
Jeff