Is this the SP version of the lens? If so, I must say, respectfully, you have not gotten close to that lens' potential yet. Take a look at this gallery of the lens, for example:
Tamron SP 300mm f/5.6 54B Lens Review | Lens, Macro, Better, Tamron, Focus | Manual Focus Lenses
Be sure to click on the "galley" link at this page. Also, the learned guys over at Manual Focus Forum have good stuff to say about this lens--you should do a search there--lots of info.
That said the lens is a slouch @ 5.6--are your images above shot wide open? Clearly, the first image is not properly focused. The lens can be bought in the US $100 range--for these reasons, is is a favorite, inexpensive 300mm choice for many. Images with the SP 60-300mm, @ 300mm, are believed to be indistinguishable from the prime 300--in case you run into the zoom pics you can use them too.