DxO Mark26 [Tested on 42 MP camera] Sharpness: 24 MP, transmission: 5.8 TStop, distorsion: 0.4%, vignetting: -1.3 EV, chromatic aberration: 9 µm. It has high sharpness throughout the zoom range, although like most zooms, it’s a better performer at the shorter end. Nevertheless, it is very usable at 300mm even wide-open — but stopping down slightly will reward you.
Handling is excellent, performance is very good throughout, albeit tailing off at the edges as we zoom in. However, it is rather pricey at the current level. Set this against its usefulness and ease of use and we still end up with an attractive proposition and a unique optic for Sony E mount users.
It isn’t capable of the same corner to corner sharpness as the twice as expensive Sony 100-400mm GM lens but it’s plenty sharp in the center of the frame and the corners aren’t all that bad just one stop down. [...] The lens is also highly resistant to chromatic aberrations, offers a relatively pleasant bokeh and is solidly constructed.
You are buying such lenses primarily for its potential at the very long end of the range which is where the performance may not be bad but it certainly doesn't reflect the promises of a G-class lens nor its high price tag. Now that being said - the center quality is still very good at 300mm. Lateral CAs are low. The distortion figures are well controlled for such a lens even without distortion correction. The quality of the bokeh is a bit of a mixed bag.