I bought the Tamron 150-600 last year and used it at the Wings Over Camarillo Air Show with my 1D4. I've been shooting Warbirds for about 6 yrs and consider myself reasonably good at panning. I've shot with the Canon 300 f2.8 IS II, 500 f4 IS II, and 200-400. Although others can pan better, I can shoot at 1/160s (@ 420mm and 560mm) and bring home a bucket full of keepers.
But...when I shot prop planes with my Tamron 150-600, my keeper rate was dreadful. I'd view the images on the LCD and saw most had a great amount of motion blur. VC was on. I increased the shutter speed but the results were still pretty bad. I turned off VC and got slightly better results.
If you check around the net you'll see other prop-plane shooters using the Tammy with similar complaints. One is a member here (BigAl, I believe).
The Tammy does not have mode 2 IS (for panning) like our Canon lenses. Some one, I can't remember where (maybe here or at dpr) said they contacted
Tamron tech support about poor panning results. Tech rep told them not to use VC when panning.
Now in defense of the Tamron 150-600 there is a flickr member who gets great results panning at slow shutter speeds with his Tammy. My hat's off to him.
But I could not get it to work, so I ebayed it. For static shots the Tammy's IQ was great. But I bought it for air shows, so I had to give it up.