The OP uses (set-frame-parameter nil 'fullscreen 'fullboth).
In contrast, I use (set-frame-parameter nil 'fullscreen 'maximized) leaving OS X's menu bar and Dock on the screen.
I do not know if Homebrew package `emacs` has been fixed because these years, I only ever install Homebrew package `emacs-mac` (a.k.a., Mitsuharu Emacs) but it used to be that one had to install Emacs.app via `brew install emacs-mac` instead of the conventional `brew install emacs` for (set-frame-parameter nil 'fullscreen 'maximized) to automatically and cleanly occupy every screen pixel except for the pixels needed for the menu bar and the Dock.
I cannot do a definitive test because I only ever use one monitor at a time, but since (set-frame-parameter nil 'fullscreen 'maximized) does not make use of the fullscreen mode introduced by Apple with Lion, it will almost certainly not suffer from the most-complained-about problem with multiple monitors on OS X.
In contrast, (set-frame-parameter nil 'fullscreen 'fullboth) does make use of Lion's fullscreen mode -- and it gets rid of the menu bar and the Dock, letting the Emacs frame occupy every screen pixel. But then at least on Lion and Mountain Lion (which is the extent of my experience with recent versions of OS X) multi-monitor support is hosed IIUC.
In contrast, I use (set-frame-parameter nil 'fullscreen 'maximized) leaving OS X's menu bar and Dock on the screen.
I do not know if Homebrew package `emacs` has been fixed because these years, I only ever install Homebrew package `emacs-mac` (a.k.a., Mitsuharu Emacs) but it used to be that one had to install Emacs.app via `brew install emacs-mac` instead of the conventional `brew install emacs` for (set-frame-parameter nil 'fullscreen 'maximized) to automatically and cleanly occupy every screen pixel except for the pixels needed for the menu bar and the Dock.