> Several investigators have found that performers tend to stretch their intervals

Johan Sundberg has excellent examples of this "stretched" tuning in some of his talks. He claims that musicians stretch intervals intentionally in order to add excitement to particular passages. The link below contrasts equally tempered and "stretched" versions of the same musical phrases.

http://www.youtube.com/watch?v=CxFVjKMsaTc&t=50m32s

To my ears the stretched versions are far more engaging, especially the example with the tenor Jussi Bjorling around 52:50.