Link time and inter-procedural optimization improvements in GCC 5

Simple and obvious question: how much % performance gain does this typically give?

It's in the article.

It depends on the application. For several SPEC results on different processors see: http://gcc.opensuse.org/

You can just skim the green text at the end of each section in the article to see the performance improvements of the various techniques.

Link time and inter-procedural optimization improvements in GCC 5 (hubicka.blogspot.com)