PCI-e is used to communicate between components in the system. For heavier loads, less time is spent communicating and more time is spent processing the data that needs to be sent out. In a sense, PCI-e bottlenecking occurs when it matters the least - when workloads on the GPU are low.It might actually be a minor issue. SLI/CF require a lot of communication over the bus. The memory in both cards needs to match up and that data is sent over PCI-e.