1.) I don't think there have been changes to the actual weather engine, though there are both freeware and payware addons out there.
2.) Thousands of X-Plane airports have become detailed and populated over the past few years. Practically all of them user-submitted! Hundreds more are released with each update. Not all airports, of course, will be populated, though most importantly the taxiway layout should remain accurate.
3.) Yes, upgrades have been made! Integrated FMC and Garmin GPS systems (where applicable) have been implemented into default aircraft (new and returning), with extraordinary amounts of detail (in my opinion, at least). You can watch the first 6 minutes 20 seconds of this video:
https://www.youtube.com/watch?v=HtJ1qdoCcds for some more detail!
4.) X-Plane 10, as far as I understand, never primarily relied on only a CPU. In my system, the GPU has always been a more deciding factor for performance than my CPU (having to do with my rendering settings), pushing my GTX 960 to 100% almost all the time! In fact, I have learned that the graphics card(s) is/are typically more important than the CPU in gaming rigs... even with X-Plane. If you feel that your graphics card is not being used to its fullest potential, you may have to adjust rendering settings accordingly (see
http://developer.x-plane.com/2007/04/cpu-or-gpu/) or you can change Nvidia Control Panel settings.
To add to that, X-Plane 11 will have a respectable strategic core loading "sequence" (as I call it), in which various aspect of the simulator are loaded on different cores.
* I am not affiliated with Laminar Research