Screen tearing while gaming
CS:GO respects its own Vsync setting, which works while in fullscreen. You can edit this in the Video options:



Enabling this setting will prevent screen tearing, at the cost of performance. Typically, this is undesirable in a competitive game like CS:GO, but there's nothing stopping you.

Beamsync is Apple's implementation of Vsync, present in the normal desktop environment and, by default, most apps. As of OS X Mavericks, the ability to enable/disable it was accessible through the Graphics Tools for Xcode:


  
  Download Graphics Tools for Xcode - Late August 2014
  Install or just mount Graphic Tools
  Open Quartz Debug
  Go to Tools -> Show Beam Sync Tools
  Select Disable Beam Synchronization
  


Note that these changes are not permanent.

At some point however, this appears to have changed: disabling Beam Sync is no longer available through the Graphics Tools. Some tools (1 and 2) claim to disable it, but I haven't verified their integrity, and neither have been updated since 2015.

My recommendation given your use case is to just keep CS:GO in fullscreen and use their implementation of Vsync, toggling as you see fit.