On XP anyway, it isn't multithreading causing the problem. It's the lack of it. XP uses a single core for the flight model and feeding the GPU all the graphics. Other cores only come into play for a few other minor tasks and when people choose rather game-like options such as other planes in the sky. Due to the way it is coded, and will be for the foreseeable future, it is basically a single threaded app when used for realistic flight simulation and other professional applications. One thing that does keep coming up with the Nvidia cards is that instancing is broken so XP has to redraw the same object over and over instead of once. At least that's how I understand it to be. The only way to run XP fast is to have a very high CPU clock for FPS and a GPU that can handle the complex rendering options sent to it. Regardless, all other things being equal, on the same machine with the same settings, FPS in Win 7 is double that of OS X 10.8...or more.
You're missing my point -- it's not the app that is multi-threaded, it's the OpenGL driver. On Windows, the OpenGL driver runs in a separate thread, which means there is basically zero time spent in the app's thread doing all the necessary work in order to feed commands to the GPU. This can lead to a doubling of the framerate, if the OpenGL driver workload can be completely offloaded to the other thread. As I understand it, the NVIDIA OpenGL driver for Windows will automatically enable the driver-level multithreading.
The Apple OpenGL framework also has an implementation of this, though it's an "opt-in" model that must be enabled (and I don't know if X-Plane enables it or not). However, in my experience, it doesn't achieve anywhere near the amount of scaling that you see under Windows, and in many cases shows no benefit at all. For what it's worth, many Mac games have a switch in the config files that allows you to enable or disable this, and I don't remember the last time I saw a significant difference in the framerates when the separate driver thread is enabled (and in some cases, enabling the driver thread actually slows things down).
So, complain about the terrible NVIDIA (or AMD) Mac drivers as much as you want, but from what I've seen, it's really just the driver multithreading (or lack thereof) on Mac OS X that is responsible for the largest of the performance differences between Mac and Windows.
Here's an example of what I'm talking about: Valve documented the issues they had with the driver multithreading when porting Portal to Mac here:
http://store.steampowered.com/news/4211/
Specifically, occlusion queries (which are very common in modern game engines) were causing unnecessary synchronization and therefore killing any performance improvements from running the driver in a separare thread.
