Mine just arrived, but undecided yet if I'm going to put both FSX and P3D on it. Sadly, I do know some of my favorite FSX add-ons will not transfer to P3Dv4.
I have both and P3D is far and away a better sim. I keep both and use FSX only for those aircraft I can't do without. I upgraded the 250 gb solid state drive with a 500 GB one and use that for P3D. The D: drive is for FSX-se and X-Plane 11.2.
Have fun. BTW, my sliders for P3D for this rig are almost all the way to the right!
Bob