I've been following his posts for some time now and MalVeauX is a far more skilled photographer than I. I'm always reading his stuff trying to glean some wisdom I can apply to my own situations. And, yeah, stopped down, my Tamron is plenty sharp. Below is a shot at f/9, 100% crop (SOOC).
I wish I had an f/6.3 100% crop of the same subject taken at the same time to post for comparison, but I don't. And using another subject under different conditions isn't a valid comparison.
That was with my 60D, BTW. Which I no longer own. I switched to a 70D shortly after getting the Tamron. Which is an entirely different subject altogether, I don't really believe another 2MP is going to be noticeable, but I kinda get the feeling that extra pixel density isn't doing the Tamron any favors either.
My lens does not do as well as above wide open though. That's just a fact, my lack of credibility on the subject notwithstanding. But, again, for $1K, I'm really just fine with this lens even at f/6.3. I wasn't expecting any better. Well, okay, maybe I was, just a tiny bit... I did hope to clearly see superiority vs. my old Sigma.
But. But, but, but... I honestly don't think it's any better than my Sigma 150-500 was, and even though I'm not unhappy with the Tamron for the price I paid - it is "as good" as my Sigma was after all and it goes to 6, which the Sigma could never do... But, still, as with my Sigma, I not infrequently wish it were better wide open and if I can find a way to pay for a lens that is noticeably better wide open at 600mm, I'll happily trade away the Tamron to get help get there.
This new Sigma represents a possible opportunity to do just that. Maybe. Or, maybe not, on both the lens delivery performance side and the "my budget" side...
Change of subject... I've tried my Tamron with a Kenko 1.4 quite a bit (taped). As already noted by the others, AF requires good light and a good high contrast target. With the extender & Tamron on my 70D In the field I never even bother trying to AF, it's 10X LV MF or nothing, for me.
After a couple trips taking lots of shots of the same subjects both with and without, I've pretty well decided the extender is not worth the trouble anyway. For me, the only time I'm breaking out the extender is for distant subjects. The results just have not been worth printing. Too far is too far. For sharing on the web, simply shooting without the extender and cropping to the same FOV yields results similar enough to make the extender superfluous. Obviously, other applications for others may yield different results and conclusions.