You are correct, it is a pcie 3.0 8x slot. If I believe what I read on the net, it makes little to no difference for a single GTX 970: 8x provides enough bandwidth. For science, it would be interesting to see if the 16x slot makes a difference. It does feel somewhat wrong to keep the card in the slower slot.
If you say it can be made to work… well, you’ve just given me the courage to try to recable the darn thing again. 😛