This week I tried Tamron 18-400 on 7D2 and M50 via adapter, at 400mm it's too soft even in the center, cropping a shot at 300mm gives practically the same resolution or maybe more, I am getting more convinced about the limits of superzooms, my walkaround zooms are 24-105f4 on full frame and 15-85 on APS-C and I am quite happy with that, I even find a 15-85 far more useful than a 18-300, since I would need the 15-18 range far more than the 85-300, but that's just me maybe.
BTW I tried birds in flight with 18-400 and focus failed miserably for accuracy, Canon 70-300 IS despite slow speed for autofocus would easily eat it for breakfast, the Tamron nailed sharp focus in 1/10th of the shots and even when focus was accurate image quality was really bad. Canon 70-300 is rather slow to focus but it's fairly accurate, sharpness tends to drop a little bit at 300mm but pictures are still usable, compared to 18-400 at 400mm the images have much more details....
BTW I tried birds in flight with 18-400 and focus failed miserably for accuracy, Canon 70-300 IS despite slow speed for autofocus would easily eat it for breakfast, the Tamron nailed sharp focus in 1/10th of the shots and even when focus was accurate image quality was really bad. Canon 70-300 is rather slow to focus but it's fairly accurate, sharpness tends to drop a little bit at 300mm but pictures are still usable, compared to 18-400 at 400mm the images have much more details....