Most every review I’ve read that compares the Oly 300mm + 1.4X to the Panleica 100-400mm claims that the former outperforms the latter. At least three such reviews tested both lenses on an Oly body, where the Panleica can’t benefit from dual IS.
While I don’t dispute the conclusions, nor have much of an issue with the various methodologies used, my own informal pixel-peeping doesn’t show such a stark difference between the two on the subject matter I shoot, and with each lens mounted on its respective manufacturer’s body. The difference appears to be no more than one would typically notice when comparing topnotch zooms and primes of similar focal length.
Both setups render images of sufficient quality to suit my purposes. And each fills its own niche in my critter-chasing world.