.... On the two points you make which I have highlit in bold :
How have you come to the conclusion that the ED 300mm Pro is sharper than the ED 100-400mm? Do you mean specifically when the 1.4x or 2x is mounted? Or in comparison with the native ED 100-400mm? I am not saying you are mistaken but am curious what causes you to say that.
I have been testing my ED 100-400mm with the 2x (MC-20) mounted on my garden's wildlife in the last few days and these RAW images do look slightly softer viewed in camera but they may be sharper when downloaded and processed - I don't know yet. I have shot over 1,000 images (surfers) with the 1.4x mounted on the zoom lens and see nothing with the naked eye to conclude that my ED 300mm Pro is sharper. The same applies to my many thousands of shots on the zoom ED 40-150mm Pro + 2x combo. However, I do not pixel peep, I simply judge each image during and after post-processing.
I may be ignorant regarding the ISO aspect but I don't understand, until someone explains, why one lens will offer better high ISO performance than another. I do understand that the camera
settings and individual image's histogram will influence ISO, as well as the camera body sensor of course. I only shoot with the E-M1X body.
This Greenfinch isn't my finest bird photo but I think it does show that the ED 100-400mm + 1.4x can deliver acceptable images in low light :
GREENFINCH by
Robin Procter, on Flickr
Underlying all of this, m4/3 sensors are currently no match for Full-Frame regarding ISO performance and it is pointless to make such comparisons and complain when someone only shoots m4/3.