is it possible following setup?
2x CSV-5300 – MST 1-3 DP Hub as 3×2 Landscape Display Group (eyefinity)
+ control monitor over HDMI output of graphic card
No i the card will not support more than 6 screens.
If you want to connect more screens you can place a second card, i would advise to use a AMD Based card for that. Or use a External USB Graphics like CSV-2300H, this is not for gaming but will give you an extra screen for standard use.
Windows can take more than 6 screens, just not your Graphics Card.
So if you have an external Graphics Device that should work.
However if that External device is not seen by the system as a Graphics Device and just splitting the output coming from your Graphics Card in the system, than it will not help. I am not sure how your system sees that Matrox DualHead2Go …