Will work perfectly fine and almost no loss in terms of performance.
https://www.pugetsystems.com/labs/articles/Impact-of-PCI-E-Speed-on-Gaming-Performance-518/
Impact of PCI-E Speed on Gaming Performance
Conclusion:
Our testing has pretty clearly shown that for gaming using either PCI-E 2.0 or PCI-E 3.0 will give you nearly identical performance. Oddly, in some benchmarks PCI-E 2.0 was actually faster than PCI-E 3.0. At the same time, x16 was not consistantly faster than x8. Again, x8 was actually faster than x16 in many cases. So unless you care about getting up to 1.5 FPS better performance, you might actually want to manually set your video cards to operate at x8 speeds - although we really would not recommend doing so.
This isn't to say that PCI-E 3.0 is not faster than PCI-E 2.0, or that x16 is the same as x8, but rather that current video cards and games are simply not able to utilize the additional bandwidth they provide. In fact, we recently showed that the performance of a Xeon Phi card is greatly reduced if you run it at x8 speeds in the blog post Performance of Xeon Phi on PCIe x8.
While we recommend using the latest PCI-E revision whenever possible, if your motherboard or video card only supports PCI-E 2.0 our results show that this really is not a problem. At the same time, if you want to install a sound card into your Z87 system but doing so would limit your video card to x8 speeds, that is also not a very big problem. At most you may see ~1.5 FPS drop in performance, but that change is so small that it is very unlikely to ever be noticeable.