So is the buffer slider in playback options different from fiddling with the registry here:
HKEY_CURRENT_USER\Software\JRiver\Media Center 12\Player Core\Secondary Buffer Minimum MS
The MC audio core uses two main buffers.
The secondary buffer (set with that registry key) determines pre-buffering and normally holds 8 - 12 seconds of audio data.
The buffering slider in options is the playback buffer, and represents how much data is given to the soundcard at once.
DSPs and other effects live in-between these two buffers. This allows MC to keep a healthy pre-buffer, but still apply DSP changes quickly. In our opinion, this is an ideal solution.
Moving the playback buffer slider is only needed if you're getting hiccups in the sound. And normally, if you're getting hiccups, it'll be from a hardware issue (IRQ, etc.) and not from MC buffering. With a modern computer, a few tenths of a second is fine for this buffer, even if you're gaming, etc. during playback.
Adjusting that registry key will allow more data to be pre-buffered, but this comes at the cost of memory usage. We believe the default system is ideal for most users, which is why there isn't a setting for this in options.