Really? Not in my opinion. I sold my 28-75 Tammy to get the 28-135. It is better in my opinion. Better contrast by far, better color by far, way better edge sharpness.
I agree with this test.
Look at the horrible Tammy corners compared to the 28-135. Better contrast on the 28-135. Look how the blacks get darker, the lights get lighter than when you are looking at the Tammy. 28-135 has rich, vibrant colors. 60 more mm of range, fast USM motor, good IS. Simply better all around other than the 2.8 speed.
The Tamron 28-75, which you have never used, is so much better optically than the 28-135 it is not even funny, so what sense does it make to recommend that he sell of the tamron for the 28-135 when he is worried that even his tamron might not be good? The 28-135 isn't even sharp to the edge on APS-C, which the Tamron at least is.
Sure you can take nice pics with any lens and if that is your budget than get it and have fun. But the scenario here is someone wanting a FF improvement over the 28-75, which the 28-135 absolutely utterly is not.