Typical GDDR5 data rates on shipping products are mid-way between PCIE 1 (2.5 Gbps per pin) and PCIE 2 (5.0 Gbps per pin), but in all cases you need an extremely wide bus to get the kind of bandwidth a modern GPU requires. An x16 or x32 bus isn't going to do the job.

The point I'm trying to make about OpenCL vs OpenGL is that if you structure the OpenCL app (or any compute app) properly you don't have to access data in the other GPU's memory very much in the first place.