When you refer to MAC's, keep in mind that the standard is just now changing to accommendate a Gc=2.2.  Earlier MAC OS's still do exhibit a Gc=1.8.  Brighter screens, better sales.  Good marketing strategy. 

Also keep in mind, digital cameras, scanners, printers and the internet have a Gc=2.2 standard globally.  But it is not an ironclad rule that your renders must rigidly adhere to that standard.  As long as you have a measure of control over your midtone range, you can tweak the settings to suite your needs within your working environment.  If the image is going to be displayed globally, then you may want to reconsider how it is rendered to conform to standards.