Looking at how I have my file system set up, /usr/local is a symlink,
and I'm wondering if using a null mount is better for performance or 
worse.

In general, I'd also like to know a little more about the stackable
filesystem stuff, but for now a short answer will do :)

Having a "real" link of some kind would be much better on me since I
wouldn't have to look at mtree complain.