I am on the other end. I have found that the image degradation is so small that it is almost negligible with a good quality filter on the lens. I believe in having UV or in the old days, skylight filters to protect the lens. It has saved me a lot of money over the years also. I somehow managed to smash two filters with no ill effects to the lens in the past. I would rather throw away 60 dollars then 400.
The purple fringe can be fairly severe, but you will only see this in high contrast areas. For example, you are shooting at a bird in a tree with a bright sky background. You will get purple fringe around objects.
I have the Tamron 70-300, and here are a few examples of what I mean.
and
Overall I am fairly happy with the lens. The price was right, it is light, well built, and the purple fringe can be easily removed with Photoshop, for those few keepers that I may want to print at a later date.
The Sigma has CA well under control.