Thanks for sharing your experience.
All sounds like good advice.
No substitute for taking a systematic approach to be sure.
I tested it on a canon 800d.
The corner stars were not great even at f2.8.
Thinking that the lens may not be the best of copies, although the other factors you highlight are more likely to be key. However there are only a few days left to return it so I might do that.
I know that when I tested my sigma 40mm the stars looked good on the dslr even wide open. When I switched to a 2600mc the quality of the stars dropped.
Cheers, Des
Can you post some samples of the images that show the coma?
Just remember that coma is going to exist on any optical instrument that has a curved lens. You see it more in the corners because the corners are the farthest linear distance from center and coma is worst the farther from center you go - which is why we stop down the lenses to begin with, so we're not shooting through that curved region of glass near the edges of the lens.
But anyways, every lens will have coma to some extent. I have three 85mm SIGMA ARTs, and they all have a small amount of coma in the corners. I also have four Rokinon 135mm, and they have coma as well.
In the end, if the photo looks good on the medium you are publishing or showing it on, that is all that matters.
You can take a look at all my images with the 85mm on FLICKR. Every one of those images has coma in the corners, but at the display size / resolution you can't see it.