After helping a customer upgrade to TFS 2015, they switched there existing build templates to the latest TFS (XAML) build process template(TfvcTemplate.12.xaml).
However after doing that, they noticed that they could no longer change the log verbosity of the build.
Starting from TFS (XAML) Build 2013 this is no longer necessary as the build diagnostics is always switched on. The easiest way to see the diagnostics is to browse to the web portal, open a build result and click on the Diagnostics tab: