I just found this topic and wanted to post my settings because in my case i got P3D4.2 running on 7 Monitors ... on only 1 PC and 2 GPUs (you can find details in my signature).
To be honest, in former times i used to have TH2Go in use (also with FSX) which was fantastic in a way. But since Nvidia released its surround mode it was no longer necessary to use the Matrox device.
My setup right now consists of:
3 x 27inch Monitors (for Outside View running on 30Hz)
1 x 24inch Monitor (Overhead Panel 2D)
2 x 17inch Monitor (Upper, Lower ECAM 2D)
2 x 22inch Monitor (connected via Y-Cable) (Main Panel 2D)
A short video you can find here:
Landing LDDU FSLabs
Video was made in January on P3Dv4.1...right now i am on P3D4.2, improvement in overall performance especially because of the Update from FSLabs to Version .237.