It looks like the situation hasn't changed since I started to look into this a year ago; data on image quality degradation, due to sensor heating in e.g. live view and video modes, particularly for mirrorless cameras, is not available. Oodles of users reporting problems and user manuals warn of image degradation, but no hard data. For me, this is a major shortcoming of manufacturers, test houses and technical reviewers alike. If this situation doesn't change when I need to replace my DSLR, mirrorless cameras will be a no-go for me.