Refactoring and performances differences between C++ versions/compilation

I would potentially need to refactor a great deal of lines of C++ code, written in C++03 and compiled with an old version of GCC. The problem is, the implementation has been hugely driven by the performances (to the point where a few lines are written in assembly).

So, I would like to know if some of you would know if and/or where to find a way to estimate a loss of performances if I refactor code using C++11/14 but with a more recent compiler.

I might be dreaming but someone may have done some test and posted benchmarks that could give me an idea...

Any help to find a good approach to this issue would be much appreciated.