Last month (Feb 22 Feb 2013) I bought the Tamron 28-300 VC lens (from B&H) for those times when I only want to carry just one lens mounted on my 5D MK III ... I am not sure if it was my copy of the lens or it is always like that, but that lens was very soft, made strange audible creaking noise when the VC & AF were being engaged and the AF point would jump after taking the shot ... so I returned it and bought a Nikon 18-300 VR lens & a refurbished Nikon D7000
Anyway here are a few images made with the Tamron 28-300 VC (mounted on 5D MK III) ... the firt and the third images are straight out of the camera ... the second and the fourth are after editing.
It's not your copy. I bought one when they first came out and you described it perfectly. The only think you left out is how slooooooow (optically) it is! It transitions from f3.5 to 5 to 5.6 and then 6.3 rather quickly in the zoom range. I also own the Canon 28-300mm beast but picked this one up thinking it would be "good enough" for casual photography. Wrong! Slow, soft and noisy...