Does anything out there even come close to saturating PCI-e 3.0 x16? All I can think of would be the enterprise segment with massive RAID arrays, SSD RAID, fiber optic networking and cluster computing.
It would be nice if PCI-e would become mainstream for SSD not just relatively expensive cards for enthusiasts. As SATA III seems to be a hindrance as the SATA spec isn't keeping pace. While Apple is the only one I see taking advantage of PCI-e for SSD in mainstream computers getting about 1 GBPS. Heck they're the only ones that seem to be interested in switching their lines to any type of SSD.
Yes and no.
Within the next few years we will see dual GPU setups that will be capable of saturating the PCIe 16 slot, and the way these standards work it needs to get ratified now if we want to see it in the next 2-3 years.
Outside of graphics there is a bit of a PCIe shortage coming up. Right now PCIe is used mostly for expansion cards... but in the near future we are going to start seeing more use of things like Lightpeak (seriously, Thunderbolt is a horrible name, can't we have the old name back?), M.2, M-PCIe, and SATA Express, all of which will need PCIe lanes. So the real question is, do we pay more for our processors and chipsets where the PCIe lanes are hosted? Or do we assign fewer lanes to each device so that we can dedicate lanes to these new IO?
Outside of the ridiculous high-end GPU space, we could be just fine with moving to a PCIe8 v4 standard for GPUs, freeing up 8 lanes each capable of 2GB/p of throughput each. That could be 4 SSDs, and a thunderbolt port right there. And for those few crazy people with far too much money that demand 4GPU setups, you can always get higher end enthusiast or workstation boards that have more lanes available.