they are not limited to pcie 2.0
whats the bandwidth in a 10gbps connection?
1gbps = 0.125 GBps
pcie 2.0 x1 = reads pcie 2.0 1 lane = 500MB/s
So the math
10gbps * 0.125GBps = 1.25GBps / 500MB/s = x2.5 or 2.5 lanes
Thats for a single lane, if you use the 2 lanes of thunderbolt
2.5GBps / 500MB/s = x5 or 5 lanes
Now for thunderbolt 2.0, its 20gbps per lane, so
2.5GBps / 500MB/s = x5 or 5 lanes
5GBps / 500MB/s = x10 or 10 lanes
however the controller only allocates pcie 2.0 x4 for tunderbolt, its a standard config procedure from intel
the problem here is if you are using the internal lcd for the thunderbolt config. Thunderbolt, doesnt pass data and image on the same lane, i.e. if you do that your bandwidth will be cut in half, since there are only 2 channels.
Currently there are several chassis that support a gpu and use thunderbolt. and one just got announced its in alpha stages
http://anandtech.com/show/7040/computex-2013-thunderbolt-graphics-from-silverstone
TB 2.0 will gives us 20gpbs per lane, and real bandwidth for us to play, at x8 there is basically no performance degradation.