I am using the FSL A320X successfully with Flyinside. The cockpit and cockpit lighting is stunning and frame rates reasonable.
There is an apparent incompatibility with Flyinside, and FsLabs say it is not supported. Fair enough.
My issue was not getting the required Spotlights (included in their package just for the A320) to work correctly.
Basically the Flyinside d3dll.fll which lives in the P3D root folder would not let the Spotlights function.
Below is a fix for it that I found - hopefully it will work for others..
------------------------------------
Run P3D "au naturelle" without Flyinside, and without the d3dll.dll ( I moved mine to desktop). Spotlights works, of course.
(Note that if you don't have a paid version of SL you need to position to LGAV to try it with other aircraft.)
Set your chosen aircraft lighting profile as required and save in the Spotlights manager.
Shut down P3D.
Put the d3dll.dll back into the P3D root folder.
Run P3D with Flyinside. You should now have Spotlights on your aircraft. The in-screen switcher menu (the one that pops up when you select "show window" is only visible on the PC screen but you can always import the Spotlights Manager window into Flyinside if you want.
In summary I have Spotlights in Flyinside - the A320 is an absolute picture at night with them working! I just "sat" on the jump seat last night in cruise enjoying the lighting!
I think that if you do a reboot you may have to do all this over again but it's a start...
Hope this helps..
Brit.