The following is my own "idea" of how CrossfireX works. I might be really wrong
Its basicly a doubling of all numbers except VRAM, which is each cards memory. This stays at the lowest cards amount for each card. What this means, is that if each card has 1gb Vram, this will stay the same also while they are crossfired. Ive understood that there is some traffic between both cards Vram when rendering screens.
If one card has 2gb Vram and the other has 1gb, both will be capped at 1gb each.
Concerning the split of PCIe lines, apparantly the difference between x8 and x16 is not a whole lot at all. Ive read differences of about 1-3% if anything. Normally something else is the "bottleneck" or there is no bottleneck because the lanes dont get saturated/filled.
Its not much i admit, but its what little i have gathered.
The downside to having multiple graphics cards in any configuration, is that although you have more power available, not every game or application will use that power. And for such multiple card configurations, both AMD and Nvidia have profiles which they update frequently.
Im sure you knew alot of the stuff i just wrote, but hey, im just trying to help out!