@unboggling - Symlinks (known as Softlinks in Unix) break if the name of the target or its location is changed - in Unix and in Windows. Given that OS/X is BSD Unix at its core, then I assume its the same.
But if you only use them for files such as metadata.db's then you're unlikely to change the name or location, and if you do, then hopefully you wouldn't do it unconsciously - i.e. you would remove the old symlinks and create a new ones referencing new targets on your SSD in the Calibre library directories (folders).
I suspect the optimal performance related preferences will depend on how one uses Calibre and one's overall work patterns. My settings (also quad core) are - Priority=Low, Max Jobs=1 and Use all cores=unchecked. My perception is that things go faster - overall.
To improve start up times I've seen the following recommended a couple of times - 'close the tag browser before closing Calibre' . But I've never figured out how to do that - does anyone know what it means? Start up times aren't an issue for me - so I never bothered to ask.