Good evening!
Unfortunately, the drivers are the most up-to-date drivers for the video card (399.07) so this is not the solution.  Changing the resolution of my 34in ultrawide monitor is also not a solution...  it is already in 1080 (height) but the width is 2560 instead of 1920.  If I chance this resolution, I get black bars on each side of my large monitor which creates a huge gap between my monitors.
So I did few testings to see if I could get a bit more of FPS.  After reading numerous documents online, I hard-enabled the SLI in Alternate Frame Processing mode and changed few other options when launching X-Plane as X-Plane does not know how to handle SLI by default.  I also increased all the graphical options to the following settings:
- Visual Effects: High
 - Texture Quality: Max (No Compress)
 - AA: 2x SSAA+FXAA
 - Number of World Objects: Max
 - Reflection: Low (only because I prefer that type of reflection on water)
 - Enabled Parked Airplanes
 
When I start a flight, the game runs really smooth with amazing graphics.  
I have noticed the FPS counter in X-Plane provides a FPS count totally different than the FPS counter provided by NVIDIA.  I'm not too sure why but the FPS in the X-Plane box  are much lower than indicated in the NVIDIA OSD (between 7 to 14 compared to 38 to 46).  I even further tested this by running the Heaven Benchmark tool and both FPS in the NVIDIA OSD and the benchmark tool were identical...  I'm not sure why there is such a gap?!?  Maybe because of the SLI???  
I've also noticed that my flights were starting with 19 AI airplanes!!!  As soon as I removed most of them (only kept 4), I gained few FPS there too...
I was hoping to get higher FPS with the rig I have but this is acceptable for now.  I will keep on playing with few options here and there to see if I could gain few FPS without losing image quality.