thankfully, unlike most tele-zooms, this lens is sharpest at the tele end of 150mm, but is sharpest at f/4
unfortunately, using the TC you get a big hit in sharpness as well as affective aperture such that centre shaprness wide open is almost half that of the much more expensive, optically superb
Panasonic Leica DG Elmarit 200mm F2.8 Power OIS lens wide open at f/2.8, so you may need to use it at effective f/5.6 to get its best sharpness with the TC (ie. at f/4.0)
minimal long CA - more pronounced at 90mm
lateral CA well corrected but doubles with TC on
no significant issues with spherical aberration but some onion ring appearance of bokeh circles
distortion level remains practically zero at every focal length if corrected in camera, but uncorrected is 2% barrel at 40mm, almost nil at 90mm and 0.7% pincushion at 150mm (almost none with TC on at 150mm)
mild coma at the edges at wider angles, minimal at 150mm even with TC on
moderate astigmatism of 7.3% (12% with TC on)
partial cat's eye bokeh circles in corners
0.6EV vignetting wide open at 40mm, 0.5EV at 90mm, 0.94EV at 150mm (this halves with TC on)
lens flare is pronounced if sun hits front glass
fast AF
build quality is outstanding, it comes with an interesting focal range, provides an excellent image quality and overall it would be difficult to find any serious flaw in it