Yep, I've used pci-e risers with my 470's and 5830s. One's I've been using are 12cm long.
Picked them up from here
$9 for 2 Lots PCI-E PCI-Express 16x Riser Card Extender Cable 1U 2U
There's tons of bitcoin miners who use them too
https://bitcointalk.org/index.php?topic=7216.740
If a 6990 or 5970 works, I'm pretty sure a 580 wouldn't be a problem. You might run into issues if you have too many gpu's running off one mobo though so if you're looking at running say 6 580's off a board, you might need to split the pcie 12v line to a molex plug.
https://cablesaurus.com/index.php?ma...30d594770c430e