I have the MFDs (and fly with TO regularly) without any fancy screens behind them. Just have them laying on my desk right between the primary screen and the keyboard and it's working fine. It doesn't make sense to put them on a secondary screen that's left or right of your primary because the natural movement to look at the MFDs should be downwards. Sure, it just needs practice but unless you're willing to build a simple sim pit with the MFDs right above your knees it's not worth to spend money on 7" USB TFTs (they are all non 1:1 ratio what makes it more odd).
If you're experienced with mouse movement while wiggling around with your TIR it's not really necessary but a nice addition. I also don't like to do heavy scripting or macro stuff within the device's drivers as it may interfere with some commands if you use certain commands at once e.g. popping flares and extending brakes while browsing in your MFD pages. No matter how careful you are with writing the stuff you will get to the point where a command is not recognized because of this. And this will mostly come in life or death situations. So better set it up in-game. ;)
I would use the Galaxy Tab as something completely different, maybe Master Arm Control Panel, UCF, CDU or Radio Control Panel. Something that has to be used more frequently than anything else and needs to be accessed quick and easy. Things will get nasty sooner or later anyways. ;)