I have read reviews like this in the past, but this is the most recent, and also more inclusive. They tested a slew of games with pcie 1.1/2/3 @ x4/8/16 using a gtx 980.
For the most part it shows that modern pcie 2@x8 or pcie 3@x4 or greater isn't going to be much of an issue for the most part (as past reviews like this have shown). However, it also shows some games taking a surprising hit by dropping down (check out Ryse: son of rome for the largest gap I have seen by dropping down in pcie slot bandwidth).
Here is a 1080p summary for those that don't like to read through: