I think it depends on what you're trying to do - generally speaking mining doesn't put any real stress on the PCI bus in terms of bandwidth, and typically bandwidth (speed) is what you're going to loose over longer cable runs.  Probably the two biggest issues you run into with cable length is signal loss and interference - so you can at least be relatively sure that a higher quality cable is going to have better insulation.  Whether or not it's going to help in the signal loss department is a bit more iffy.

I've literally built 100's of rigs using those exact 3ft'ers I've linked to without any issues (you may have seen my custom miner enclosures, they all use these cables).



Personally I hate this design of risers - it does just about everything wrong in my opinion.  First, the molex and PCIe connectors are located such that they almost always interfere with the card or sit right up against the heatsink of the card, which it terrible in my opinion - personally I only buy the ones with the 6-pin PCIe connector for power, and then just use an adapter if I need to get it from something else.  The slot connector has a right-angle plug on it, so on smaller builds with just a couple cards on a traditional motherboard it won't be an issue - but on a larger board like the Asus B250 Mining Expert you can't use these because the small slots all overlap - only the typical upright ones work.  Beyond that it's fine.