Tamron 28-75 bought new: ¿worth calibration?
I'd send it in. I have had two copies of the 28-75, and the difference between a good one and a bad on can be pretty big - the following two images were both taken at f:2.8, the first with a non-BIM copy of the 28-75, the second with a BIM
Both lenses were carefully set up with Reikan FoCal on my D300.
A good copy of the 28-75 can do very well at f:2.8.