A PCIe2.0 card will only run at PCIe2.0 even if you use a PCIe3.0 slot
Also, you need an Ivy Bridge processor to make use of PCIe3.0
Even though 3.0=2x2.0, thats on a per lane basis
The bandwidth wont be reallocated among more lanes like you are expecting it to be
Also, the performance difference is supposed to be minimal between 8x/8x and 16x/16x so I doubt you should be worried because of that