It seems to me that if you are running a Crossfire setup the bandwith on PCI-E 3.0 8x is identical to 2.0 16x.
Not at all. You got it in the title: PCIE 3.0 @ 8x = PCIE 2.0 @ 16x.
Also no graphic card to date does max out the PCI-E 2.0 16X lanes, almost any card on the market doesn't even max out PCI-E 2.0 8X lanes or PCI-E 1.0 16x lanes. You could get the same performance from a PCI-E 3.0 card into a PCI-E 3.0 motherboard @ 4x, as you would get from and PCI-E 3.0 card into a PCI-E 1.0 motherboard @ 16x