You are 100% correct, for both. Keep in mind this is how crypto miners get their work done: they don't need the bandwidth so they use PCIe multiplexers and split the lanes into multiple x1 slots. I actually wouldn't be surprised if you could just chop off only pin 17 of the PCIe connector and suddenly you'd get an x1 slot, with the rest of your lanes will becoming useless. But, pins 31, 48, and 81 might also have to be chopped off. But like you said - not recommended lol. The only reason I can think why a GPU would have x16 slots nowadays is for backward compatibility. So take a GTX 1070 for example, where you should be able to run that on PCIe 3.0 on x8 lanes and hardly ever see any performance loss. Run it on x4 lanes and you most certainly will. Remember that bandwidth is roughly doubled after every PCIe generation. So, if you stick that GPU in a PCIe 2.0 x16 slot, it should pretty much run at full speed. You're probably saturating almost all of the available bandwidth, but it will run at full speed. Stick it in an x8 slot and you will see a performance loss, since that's equivalent to a 3.0 @ x4. However, we're at a very good point, where very few products can saturate PCIe 3.0 @ x16. Since such motherboards already exist, to me, now is the chance to phase out x16 slots for next-gen PCIe. 4.0+ GPUs could still be made to support x16 lanes (the excess lanes don't have to be used), but to me, motherboards should start looking to downsize.
Most of the time, it's the exact opposite: heavy compute loads tend to need fewer lanes, not more. So yes, I take your point that certain situations would call for that, but in such situations, you're probably not strapped for 32GB/s of throughput.
