Regarding pcie clock itself then at least on Intel boards it goes as high as bclk. On SB you can count on about 103MHz, on IB about 105-107, on Haswell with some luck up to 110. I mean stable as max is slightly higher.
At the same time some HDD will generate errors or shut down when you pass 105MHz, some other will work without issues up to 120MHz pcie. Generally SSD have higher tolerance for high pcie but still I see no point to overclock it.
If I find some time this weekend then I will try max on i3 4330 / ASRock Z87M OCF.