[davele]

On initial release most vendors ship the "reference" card. After a few months they bring their "Value-Add" card to market.
It is these 2nd generation cards where they add unique functionality & capability to their cards.
From what I've seen the "FLEX" versions of Sapphire cards have by far the most flexibility & capability for Eye Infinity. ie: supporting 5 VDU's when the reference model only supports 3.

It is definitely not a 5 Series or 6 Series thing. More Monitors demand i) More GPU speed, ii) more RAM & iii) more power for more Outputs. As you add this to a card the cost, heat output & power increases. Thus the low end models in any series card will not drive as many VDU's as the higher end models.

Why don't they offer more DVI?
An advantage of Display Port is that you can power 3 monitors with the same number of transistors that 1 DVI-Dual Link requires. (As DP doesn't need individual sync signals.) That is why cards which support 4+ VDU's do so via DP.
Also the recent DP 1.2 spec permits serial connection of VDU's this will mean in future one output of the card could drive 3 monitors each. Perhaps more VDU's in future if they move to faster fiber optic connectors.

PS: The 7 Series cards are likely to support even more. The 28nm fab means lower heat, lower power, so more room for custom extras.