AMD Xfire doesn't work/scale nearly as well as SLI. Also, SLI is more beneficial than Xfire because NVidia is hampered by low bandwidth but high stock speeds. SLI aleviates that issue and the faster clock speeds make SLI scale much better. 850/1200 @ 512Bit VS 1100/1500 @ 384 Bit 512 bit won't help rly much at all over 384 bit at this range.
That is a myth, that's not how Crossfire/SLI work.
Textures and shaders need to be loaded on both buffers at the same time, so the active buffer of 2 256 bit cards in SLI/XFires isn't 512, it remains 256.
Same thing with the 650 Ti boost cards, the active memory buffer will remain 192bit because the memory load doesn't split between the cards, it actually doubles because the same load needs to go into both cards.
Crossfire and SLI scaling depends largely on the game and this is evidently clear in Linus's Titan review as the SLI 660 Ti setup failed completely in one game while the Asus Ares II failed in another game.
Numerous reviews also show that two 7970 Ghz edition cards in crossfire generally outperform 2 GTX 680s in SLI.
Scaling in SLI isn't generally bad nor is Crossfire scaling generally bad , each is just not as good in some games as the other.