MhzManiac
You don't really receive double bandwidth with 3.0, but the new addressing uses far less overhead giving you better throughput. 
I'm not sure if the gains would be worth another cpu. I did exactly that to get my GPU's up to 3.0. While it did improve things, it was only about 10 -12 % faster than when I had the Sandy running at 4.8 Gig. 

While that is true, you really will not see the full extent of the speed of PCIE3 with GPU's until the GPU can max out the Gen3 slot at 16x, or very close to it, then set the PCIE slot to Gen2 in BIOS and rerun the test, I am sure you will see a much larger difference than 10-12%.  However I do doubt it will double in performance even then, but it will be substantially more than what you are seeing now.
 
 
For the comparison on M.2 directly, look at the difference between M.2 on an X99 that runs from the PCH (Gen2) versus either a board that pulls lanes form the CPU, or a Z170 as their PCH is Gen3.  Look at those speed tests with the same model drive and OS and that will likely give you the most accurate difference.
 
Also, Gen2 M.2 is still definitely faster than SATA SSD's.