Just for fun, I looked around on one of my adventurings for some sort of object a bit of distance away, to try comparing the quality of the Nikkor 300mm f/4D AF-S and the Tamron 28-300mm f/3.5-6.3 VC. Given the latter only goes as wide as f/6.3 at 300mm, the exposure was quite a lot longer at 1/400th, versus 1/1250th for the Nikkor - so there's technically the possibility of some motion blur involved, though the Tamron's internal stabilisation should've taken care of that. It's only a quick test, therefore, but it does nonetheless give a useful indication of their relative merits.
These are, I should note, 100% crops - no rescaling involved, just extracting a particular portion of the image at original resolution. Not a subtle difference!
I was amused to later realise said construction was about fourteen miles away. =:D