In the meantime, I found a publication that gives very clear and convincing data regarding my question. Though the protein complexes described there are no isolated enzyme complexes but a different kind of protein complex, this publication sheds light on the potential of the technique of up-to-date cryo electron microscopy. For an ion channel, a resolution of 8.8 Angstrom was reported, resulting in a final map of 3.4 Angstrom resolution. The article is by Richard Henderson: Structural biology: Ion channel seen by electron microscopy. Nature 12/2013, 504, 93-4, The article refers to most important articles, e.g. by Liao and colleagues that are the basis for Henderson´s article.