I'm not entirely clear on how much bandwidth mining really needs, as it seems lots of people report nearly full MHash out of x1 slots, so if you could find something that was 16x more price efficient (4x double slot PCI-E lanes for 1/4 price) that'd be a very intriguing find. Not sure where the price point is for it being worthwhile, but I'd say that's pretty close there.
x1 slot is all you need for mining. It uses way less bandwidth than even an x1 slot provides.