So lenstip tested the olympus 300mm lens and in the review they make comments comparing the two lenses including defending their results for the Leica.
Basically the 300mm is over 70lpm wide open. They also test it with the 1.4 converter.
Unfortunately I had to start a new thread due to some foobars forcing the old thread to be closed; please behave. Btw imaging resource also has a review of the leica 100-400; and while they state the 300-400 range is 'sharp' they show in their 'blur' test that is not nearly the quality you obtain at 100-200mm. My guess is that max aperture of f6.3 @ long end is causing issue and perhaps outdated firmware (there has been report that firmware update actually has a big impact).
In any event while I think the results are a bit low i can't really see the results being spetacular @ 400mm.