In theory diffraction limit for APS-C 16Mpx sensor is at ~7 aperture ( depending on wave length), but in practice SW80 ED with 1,7 TN ( aperture 12,5 ) gives better reach. I stack 1,7 TN and 2x TC only for video- goshawk was so cooperative, air was very clear and light nice - an I was able to try different combination. In normal condition such stacking is useless due to high ISO noise and camera vibrations/air turbulence.
Another member of Accipiter family female Sparrowhawk perching on a tree 60 m from my garden.K-5, tripod, ISO=640, 1/400s , 1000 mm . Original frame cropped to 50% linear size, denoised in NeatImage- exif is gone.