Question about PCIE Gen Setting

PCIe Gen 1 x16 = PCIe Gen 2 x8 = PCIe Gen 3 x4

So by running in Gen 1 mode you are basically running at PCIe Gen 3 x4 bandwidth. There should be no problem with the R9 390X but I can't say for certain if it will bottleneck the GTX 1070 or not. I would hazard a guess and say it shouldn't. I do not have a 1070 or 1080 to test with so I can't be 100% certain. I do know that prior to the release of the GTX 10XX series there were no GPUs capable of fully saturating PCIe gen 3 x4 bandwidth. 

I hope this is what you were after.

I agree with you, I'm not having a BIOS problem, never even hinted that I did.  It's a cable issue, in that the cable somehow is not able to let me run the slot in Gen3 mode (no matter what cable I use).  And again, let me repeat myself, I'm not asking for a solution.  I asked a specific question about what happens to the slot when you put it in Gen1 mode.  Thankfully, the question was answered by another user who does know how to read for comprehension.  We can all go back to our regularly scheduled program now.

Risers have been around a long time, many of which are old and only able to work on a certain PCIe Generation. Some require gen 2 others gen 1. In some cases they simply have compatibility issues with certain motherboards, in many of these cases using a powered riser as Wardog mentioned proves to be the solution. 

To answer your question again:

If you are using an x16 riser on the x16 slot and have the slot set to Gen 1 then you are operating at the equivalent of PCIe Gen 3 x4. If you are connected to an x8 slot then it would be equivalent to PCIe 3.0 x2 and PCIe Gen 3 x1 if it is connected to an x4 slot. 

This is because:

PCIe Gen 1 = 250MB/s bandwidth per lane

PCIe Gen 2 = 2x PCIe Gen 1 bandwidth

PCIe Gen 3 = 2x PCIe Gen 2 bandwidth = 4x PCIe Gen 1 bandwidth

See this breakdown for more info.

If you look at the benchmarks you will note there is very little difference between the 3 generations in actual gaming metrics. We are talking at most 3 - 5fps between gen 1 and gen 3 on a single GPU system. This could be considered within the margin of error for these benchmarks. The more telling evidence is the frame variance graphs which show all 3 tests pinned almost exactly on top of each other. If there was a bandwidth bottleneck then there would be sharp spikes peppered along the graph where this was happening. The reason for this is that current GPUs (at the time of that review) were not capable of fully saturating PCIe Gen 1 x16's 4000MB/s bandwidth. Now if you look at the little table above you will note that that equates to the same throughput as PCIe 3.0 x4 or PCIe 2.0 x8.

Now without bandwidth details on the GTX 1070 I can't say if it will be more or less effected. On one hand it is a significantly faster GPU than the GTX 980 used in the review I linked but on the other it almost certainly uses more advanced texture compression and other algorithms. This means it could go either way, it could use more or less bandwidth than the previous generation depending on how Nvidia optimized it. My guess is that it will be more or less the same.