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I found interresting tests and comments at http://medfmt.8k.com/mf/hassylenses.html
page 9 is about Komura teleconverter. I do have one and I am still testing with a F/4 150mm. So far results are quite acceptable even though less sharp then what I get with the 150mm alone.
I got my used Komura for 100Euro, prices sometimes fly up, I recently found a used one for 320euros.