I know there isn't really any performance difference in games between PCIe 2.0/3.0 and/or x8/x16, but is this also the case with oclHashcat, or can it actually make use of the extra bandwidth?
AFAIK there have never been made any benchmarks, or published. People say it does not make any difference.
Yes, it's been reported several time that you can use x1 for cracking without problems.
There's no difference between PCI-e 2.0 and PCI-e 3.0.
Going from x16 -> x1 you will experience longer loading times, but the cracking speed itself is essentially the same (there are some caveats to this but it gets rather complicated and involved.)