Afterburner

Here is what I can tell you from my experience:
 
Let's suppose that you run identical settings and that you initially have all shadows deactivated in both sims.
If you monitor the FPS immediately after loading the simulator, FSX:SE will display a considerably higher frame rate than P3D. However, if you switch views and start moving the view angle from the spot view, the FPS in FSX will drop and move closer to P3D (though it will be still higher).
 
On my system, as soon as I approach a more complex airport, the FPS in FSX drop even more compared to the initial loadup (sometimes by over 50%), while P3D maintains a more consistent frame rate during that stage. So I don't think that your expectation that P3D's FPS will drop by half during approach will turn out to be true.
 
Now here is where you gain from P3D: As soon as you start enabling all kinds of shadows (aircraft, VC, autogen, scenery objects, etc.) and water effects, the FPS in FSX will drop dramatically, because these features are very CPU-heavy (even under DX10), while your GPU is bored to death. This gets even worse with CPU-intensive airplanes (PMDG). In P3D, it's your GPU that is taxed, which will give your CPU still enough breathing room and leave you with more FPS despite better visual quality.
 
So in a nutshell: If you keep the eye candy deactivated, you are able to achieve higher FPS in FSX than in P3D. However, these FPS are not very consistent, since they can drop heavily during an approach. As soon as you activate shadows and water effects, P3D will win big over FSX in terms of FPS.

Alright, I was successful in identifying and solving the cause.
 
After having completed and saved the first half of a long-range flight from Dubai to Perth with the LVLD 767 yesterday before going to bed, I played around with the menus (right-clicking and switching views) and got another CTD quite soon. This time it was a BEX error. Next day, I continued the flight and made sure to use the menus as few times as possible. I made it to Perth fortunately (and found that I had no parking options available from ATC ground because there were no gates for heavy airliners - but that's another story). After having parked with no more intentions to fly, I switched views many times by right-clicking on the mouse, and the Crash to Desktop striked again with the mysterious BEX error. I was convinced that the crashes were related to accessing the menu, which are known to be triggered in conjunction with the uiautomationcore.dll file.
 
Next, I loaded the default Trike on a default airport and "combed" through different views via right-clicking. I switched every available view in order: Cockpit views, outside views, runway views and views from AI planes at different locations for a real stress test. Even before I could try the AI views, I got another crash with another BEX error. Deleting the uiautomationcore.dll file didn't help. Then I wanted to try to replace the existing dll file by an older version (13565 instead of 18000 I think). Tried again, but with no success.
 
Having tried with and without all available versions of the uiautomationcore.dll file to cure the CTDs unsuccessfully, I was out of clues on what to try next after even uninstalling some add-ons (GSX), and decided to roll back to the boxed version, which never gave me these issues. The slightly better performance under heavy load and the reduced stutters of the FSX:SE edition weren't worth the risk of having the program crashed in my opinion. Before I had uninstalled the FSX DVD edition with all the add-ons to make room for the Steam edition, I backed up the system image, which I now rolled back to.
 
Well, guess what: the "menu stress test" under the same scenario lasted longer, but the FSX DVD edition eventually crashed as well. Once again it was the ominous BEX error. I didn't expect that. Something was wrong.
 
After having tried different settings and setups, I was finally able to pinpoint the culprit: My Ant-virus program (Webroot). I don't know why, but it caused the crashes. After having disabled it, I got no more crashed to desktop from switching views through the menu even with putting a lot of stress from switching quickly at different AI plane views. When I made the tests on FSX:SE, I had the anti-virus program always activated. Since it is most likely THE lightest and least intrusive anti-virus defender available, I didn't expect it to be responsible for menu-related crashes, more so as it hadn't happened before. But maybe there was an update to that anti-virus program that changed things...
 
Anyway, I had fortunately created another system image with the Steam version before uninstalling add-ons and rolling back to the DVD version. Now I wanted to go back to the Steam Edition, because I was sure that I needed to deactivate the anti-virus program to avoid the crashes - and I was right!  After having conducted another stress test on the Steam version, no more crashes!
 
Thank you for all your suggestions!