I've done an unscientific test of all my 200mm capable lenses. Certainly not up to Gub's standards, but enough for a quick evaluation. All taken RAW on tripod, IS disabled, 1/500s, TaV, Centre weighted, F/11 since that's apparently where the DA 55-300 lenses are at their best, but I don't have one to evaluate, but thought I could try a focal length I do have plenty of that's within the zoom range.
Flower was about 10m away. Any images that looked particularly bad, I repeated a few times in case I could get better results.
Images imported into Lightroom, cropped, exported with no other changes and compiled in Photoshop.
Noise varies as lighting was changing a bit.
Google Photos degrades image quality a bit, but the comparison between lenses still is clear enough.
I'm actually surprised how good my old Tokina 28-200 is compared to newer lenses. In my film days it lived on my camera, and produced some nice images, and although it's inclined to halo a bit in bright conditions, it's certainly not the worst lens in the lineup by a long way, and it has a pretty handy walk-around zoom range.
I'm surprised how poor the big Sigma is, even at 200mm.
Link to full sized image.
Google Photos