I agree, from what I've read, there is plenty of overhead bandwidth available in PCI-E 2.0 still. They even did away with Crossfire cables and routed over PCI-E on the R9 290x's. Tells me there is some bandwidth left.
Speaking of, if you want pci-e 3.0, you might want to look at the AMD line of APUs and the Crossblade Ranger 2 mobo, I "THINK" it has 3.0
-hax0rmort
MOBO - ASUS ROG Crosshair V Formula-Z
CPU - AMD FX-9370
RAM - AMD Radeon R9 Gamer Series 16GB (2 x 8GB) 2400 MHz
SSD1 - Samsung 840 EVO 250GB // Boot
SSD2 - Samsung 840 EVO 250GB // Game
GPU1 - ASUS Radeon R9 290X
GPU2 - ASUS Radeon R9 290X
COOLER - Corsair H100i AIO Liquid Cooler
PSU - Corsair RM Series 1000W Gold PSU
AUDIO - Creative Sound Blaster Z
CASE - NZXT H440 Red/Black
KEYBOARD - Corsair K95 RGB
MOUSE - SteelSeries Sensei
HEADSET - Logitech G430