AlanF said:
I own both the Tamron and the Canon 300/2.8 II + TCs and can say from considerable experience with both that the Tamron is a "respectable" 600mm. Remarkably, the Tamron stacks up very well against the 300/4 from Canon as seen on TDP and MtF measurement on Lenstip and is far better at 400mm than the 300/4 with a 1.4 xTC
My comment was about
600mm if he needed that focal length--most amateur zooms do better when not racked out, and from what I've seen of posted photos I don't consider the Tamron consistent for anything more than about 450mm. Beyond that the color and contrast are too low for me, which is the same reason I stopped using a TC with the 300 f/4 (except as a spotting scope). I would consider the Tamron for the same application I got the 300 f/4 for, which is telephoto for backcountry backpacking trips where weight is a limiting factor.
With tele zooms, most amateurs want to shoot at the longer end more often than not. Unfortunately, the typical amateur expects a "really good" X --> Y tele zoom to be good at focal length Y, when it is typically good only at focal lengths X --> (Y - (Y-X)/4).
Your post doesn't state clearly whether your Tamron is equivalent to your 300 f/2.8 II + TC at 600mm. I would be surprised if the Tamron did that well, but if you have some side-by-side comparisons to share I'd be interested in seeing them.
I do wish a manufacturer would step up with a 250-500mm or 300-600mm that is tack sharp with rich color at the long end. 2x ratios seem to be more manageable lens designs, and 500-600mm would be enough for most birds and other wildlife.