That’s correct. When you buy a Mac, you don’t need to download extra drivers. Only if you buy a Mac Pro and later install a different (or additional) PCI graphics card, you may need to download drivers.
The only PCIe expansion capability available on any mac (including the new mac pro) is thunderbolt. The new mac pro does not have any internal expansion.
You can however, use something like [this](http://www.sonnettech.com/product/echoexpresschassis.html), as long as you realize that you only have access to PCIe x4 bandwidth.
I've also not seen any posts of anyone actually doing this yet.
Nominally these are to support higher end cards for things like CUDA and OpenCL. Interestingly, though, the official drivers contained in updates from Apple have evolved support for pretty much every nVidia card since 2010, and recently starting gaining support for the AMD 7xxx series. For example I run a hackintosh with a fully enabled GTX 470. In Snow Leopard days I had use the nVidia supplied drivers for their workstation cards, these days it Just Works.
Yes, I'm sticking to Mac OS X
For example, OSX has really bad (read: no) support for hardware slightly deviating outside of its niche. It also uses an arguably-broken UI metaphor, and has no truly good package management story.
While it is well known that Linux supports the most hardware devices out there (in absolute terms) and Windows is most likely to support currently common hardware, neither allows one to rely on a recent version of OpenGL being available.
On Windows one must install drivers to get OpenGL support because GPU manufacturers focus on the very platform-specific Direct3D at the expense of everything else, on Linux support is spotty and only through binary blobs because the same GPU manufacturers don't provide full open source drivers for the hardware their customers paid for.
I was lamenting the state of standards compliance in general and OpenGL in particular; in this particular case OS X does pretty well.
Suppose I had GL 4.3 capable hardware.
Closed Source Drivers on Windows = OpenGL 4.3
Closed Source Drivers on Linux = OpenGL 4.3
Open Source Drivers on Linux = OpenGL 3.1 [best case]
Closed Source Drivers on MacOS = OpenGL 3.2
My Game Engine Requires = OpenGL 3.3
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As you can imagine, that means:
If you're running Linux on open source -> no
If you're running Mac at all -> no
In fact, what is ironic is the ONLY systems in the ENTIRE PLANET that can run this game are ones using proprietary drivers, specifically Windows and *NIX (but not MacOS)
Windows also generally will try and install driver updates as soon as they become available--this hardly makes OGL some magical burden to keep up-to-date.
If anything, OSX has been lagging behind hard until recently.