I use Spacedesk since some days with a Samsung 10.5 tablet and can confirm it's a great tool as I can move my Pilot2Atc plugin on the separate "tablet screen" (just as with a second monitor).
But as I fly with Prepar3d, I have to start the simulator completely before I make the connection from the tablet, else the simulator exits with a "cannot initialize DX11" (or similar). Don't know if this is an issue for FSX.
Also, Spacedesk knows only two resolutions: the native tablet's resolution and a minimal resolution that defaults to 1024x768 but can be changed in the settings of the android Spacedesk client. I changed it to half the screen size of the tablet (1260 x 800) to avoid aliasing effects.
I checked the network transfer rate while doing some actions on the tablet screen and was surprised that only some KB/s to low MB/s were transfered, it seems, Spacedesk does some compression or transfers only the differences. Regarding CPU it uses 10-20% of a 4790K core.
