That is puzzling. Odd for a prime to be beat out like that.

I get results like your owl if I try to shoot my 300mm with the 2x wide open. Results improve dramatically when stop down 1 stop (that's why just about everything I post using that combo is at f/11).

But you're already nearly 2 stops down from WO. Have you tried any side-by-sides using a range of apertures to find the optimal resolution setting for that particular model? Can't imagine it being more than 2 stops down but perhaps it is. Whatever it is, it would be a good idea to know so that you can always use that as your "default" aperture.