I've owned a Canon FD 500mm mirror, plus a couple of different Russian 500mm lenses (and a 1000mm), and for sharpness the Russian lenses win hands down. How sharp? Not as sharp as a 500/4.5L Canon, but much sharper than any manual focus zoom lens I've used with any teleconvertor (and I've owned all the Canon teleconvertors).

If you can find one the old Vivitar solid cat lenses were also very good.

Personally I would look out for a fixed 400mm lens - I've used a Sigma f5.6 400mm with good results - much sharper than azoon with a TCV..

Another alternative would be an old Pentax 500/4.5 screw mount lens with an K mount adaptor. I've seen them priced pretty reasonably..

I just had a quick look at KEH - they list a few 500mm mirro lenses with T Mounts for under $70.....