A few weeks ago, I wrote about problems with kernel 3.13 on Ubuntu 12.04 LTS and 14.04 LTS.
Most likely, the problem that caused the excessive CPU load and occassional high network latencey has been fixed by now and the fix is going to be included in version 3.13.0-33 of the kernel package. I experienced this problem on a multi-processor machine, so it is probable that this was the problem with KSM and NUMA that has been fixed.
I am not sure, whether the problems that I had with IPv6 connectivity are also solved by this fix: I had experienced those problems on a single-processor (but multi-core) machine, so it does not sound like a NUMA problem to me.
Anyhow, I will give the 3.13 kernel another try when the updated version is released. For the moment, I have migrated all server machines back to the 3.2 kernel, because the 3.5 kernel's end-of-life is soon and the 3.13 kernel has not been ready for production use yet. I do not expect to have considerable gains by using a newer kernel version on the servers anyway, so for the moment, the 3.2 kernel is a good option.