Have you tried it without the converter on a tripod or similar steady surface.
A 500mm lens is difficult to hand hold at the best of times, especially with manual focusing and a converter would make this virtually impossible. x2 converters only work well with really top notch lenses.
An F8 lens will require good light.
This lens is being discussed on this site
OPTON 500f/8 lens so it might be worth going there for advice.