why 16x4x does dont hinder performance compared to 8x8x is because in crossfire most of the data comunicated to the second card is via the crossfire connection, not the pci-e slot.
oh btw, about the 25fps thing, our eyes don't see in frames per second, our eyes percieve images in an "analogue" format, not a "digital" one, aka, we are constantly seeing things that blend into each other, not seeing heaps of different pictures wiht transition pictures inbetween, on a TV, they have transition pictures which give the perecpetion of smooth vision, but in real life, the human eye can perfcieve up to about 70 different changes in light per second, so you can percieve about 70fps. btw, everyoen is unique, so this figure may vary.