For what we know the only updated thing on th 2015 is the ios support.Probalby nothing more or something important as it was not documented by them.If you look at the udk games made in this forum>most of them use a udk 2012 to 2013.Thats why i said i beleve the 2015 may not be of much importance.
Here is some info
By the way,what does exatly mean Windows signing support for 64-bit fat binaries?