This article is outdated and kept only for archive purposes.
As you may know, iXit overlay for Gentoo offers MultiPath TCP support for well tested Gentoo kernel 3.11.10 with MPTCP patchset from stable branch 0.88.
Since today, you may find new mptcp-sources (link invalid), based on 3.12.2 kernel with MPTCP patchset from trunk branch. It's lightly tested. No regression found for our systems.
Enjoy tuned kernel 3.12, it should provide improved TCP stack and better performance for modern AMD GPUs.