Test of the Nikkor 800mm f/5.6 AIS EDIF + TC20EIII (=1600 f/11).
Shot with D7000, tripod, ISO 200 f/16 (=8) at 1/160s. ~ 20% cropped. ~200m of distance.
Test shot of the old Nikkor 800mm f/5.6 + TC20EIII = 1600mm f/11.
D600 + 800/5.6 + TC20EIII + tripod + remote shutter, ISO 320, f/22 (f/11x2) at 1/200s.
V1 + FT1 + TC20EIII + Nikkor 800mm f/5.6 ai-s EDIF + tripod + remote shutter; ISO 800, f/11 or f/16 at 1/80s. Due to the equivalent 4320mm tele, I had to make a 3 shots "pano".
Old test of the new TC20EIII on the old Nikkor 800mm f/5.6 AI-S EDIF.
D300, ISO 200, 1/50s at f/16(=8), tripod, MLU during 20 s, cable release. Spot metering, -0.33 EV.
Small pass of smart sharpen was applied, amount 50, 0.2 px.
See that I used a 2x TC, so image with naked lens would be better.
Even in the less probable case you do not like the lens, surely you can sell it by (at least) the same price you bought it.