I am fortunate enough to own the wonderful Canon 400/4 DO and a 500/4L, both of which are used a lot of the time with a 1.4x and yet I just bought the Tamron primarily for one reason: a single lens solution when traveling to places where taking a lot of gear would be difficult. My current bodies are both 1D Mark IVs.
I tested the Tamron for several days on a variety of subjects and then sent it back. For stationary birds, and objects, it performed admirably with really excellent sharpness,and contrast. Wide open @ f6.3, it was okay but not great. Images greatly improved when stopped down to f8. It does need a LOT of LIGHT to perform well.When I (tried to)use it for BIF though, it failed miserably... regardless of the focal length, it would not track properly, and the AF continuously hunted for focus. I could never get it to lock focus despite using a variety of focusing techniques.
Frankly, it's a very nice lens with an excellent build quality and superb price point but unfortunately, it didn't work with my camera bodies. If Tamron fixed the tracking issues to enable it to work on a 1D4, I'd reconsider purchasing it.