One of our members at this site, Ammadoux (her library:
Media added by ammadoux) for years were claiming the exact opposite. I have only used the pana version so do not know. My photos with the pana 100-300 start at page 19 of my images in birdforum:
Media added by njlarsen - this was with the GH2, a by now 10 year old version.
I disagree with this because to me it is not a quest for being noise free (if you want that, a medium size sensor and a 1000-5000 mm lens is probably what you need), for me it is about achieving a noise level that allow you to make photos that are acceptable even when the light is low, at least when the bird is important for you. The location for my antshrike was probably less light than anything you would find in the US 90% of the time.
By the way, here is another iso 6400 which I believe had a better exposure than the one previously linked to:
Unknown butterfly
Niels