With a multi-monitor setup one is fully dependent on the ability to move instrumental windows to other monitors.
In FSX this is a straight away action with no particular loss in FPS. But with P3DV4 one can't just move instrumental windows straight away - before allowing this one should UNDOCK the instrumental window. The problem with UNDOCKED windows in this case are that they steal away around 15-20% FPS from the overall scenario!

Would there perhaps be away to overcome this annoying issue by editing some *.cfg and make instrumental windows work as they do in FSX?   

Cheers 

Have a look at this topic I started a week or so ago

YMMV as I am using a 4K monitor, set to 30hz refresh and woth Vsync and triple buffering set to on in P3dV4. In my case I found this drop in FPS was due to using an external frame rate limiter (NI) (prior to me changing to 4K and using Vsync).

https://www.avsim.com/forums/topic/513687-p3dv4-multimonitor-undock-windows-halves-frame-rate/

Rob

Thanks for the link Rob. Interesting aspects there.
However in my case I'm using 2 x AMD Radeon boards for 8 monitors - Not nVidia and no external frame rate limiter here. Anyway will try and experiment with my board and P3D4 settings, principally, based on your experience.

But really, something must be wrong with P3D4 dealing with undocked windows, when this issue isn't present at all in FSX.      

My problem is now solved, simply by turning off Vsync and triple buffering in P3D4!