You have a P***ing contest going, there, with the guy who sold you the lens and that is unfortunate. Going forward, if you are going with non-manufacturer lenses, I think you have gotten the inside scoop on what to shy away from.

That lens chart on the Dyxum forum will be a good guide on what to expect from almost ANY compatible mounted lens. Check there first and then ... plan your finances.

Let us know where it goes from here.

Good luck.