This is how it would work if PCI-e bandwidth were the major limiting factor of graphics card performance. However, this is not the case. No single card is bandwidth limited on PCI-e 2.0 x16. Therefore, no single card is going to be limited on PCI-e 3.0 x8.
So, you could crossfire some 7970s on a PCI-e 3.0 setup, with both running in x8 mode, and have basically no performance loss compared to two cards running in PCI-e 3.0 x16. Even in PCI-e 2.0 x8 with two cards, you're going to see very minimal performance loss (and I'm thinking about 2x7970s here - you wouldn't see any loss on midrange cards).
PCI-e bandwidth really isn't a limiting factor on GPUs.
This may be dated information. My understanding is Crossfire runs well even on 16x/4x Crossfire (provided the 4x is not compromised by shared usage).
SLI is reputed to require 8x/8x for proper performance.
There is a toms article on scaling with AMD vs Nvidea GPUs with AMD vs Intel CPUs, if memory serves.