I have zero experience onthese type of lenses however from what I've read, mirror lens are not much good. In photography, you pretty much get what you pay for so don't skimp on the money when getting a lens.
As for the Kenko teleconverter's, I've had two but I never used them much. They only work with the higher end lenses and even then, theres a chance the lens will not autofocus. I hope this helps, albeit, slightly late!