Could very well be hardware- or driver-dependent, and obviously the switch or device on the other side of the cable has a say in it too. There's an awful lot of pluggable stuff in there to support everything in the market. Really this thing needs to say "I only measured 1.3Mpps between two 3.11 kernels on IVB boxes with RTL8168 devices connected via a Netlink blah blah blah switch" to have any diagnostic meaning.