The problem was more using a 300mm lens with an unknown teleconverter of some... variety.
It seems impossible to accurately focus the lens in this configuration, as the image is always soft no matter what you do.
Also, foreground and background bokeh seem to break up into their constituent light waves with this setup. It's far worse than your ordinary chromatic aberration and purple fringing. Twigs turn into a yellow and purple blur.
The magnification is pretty significant though. Could be more usable for video rather than stills.