As an alternative to consider the Canon 100-400 MK2 is a cracking lens, pin sharp and fast to focus too. I recently bought a 7D2 so I haven't, as yet, had much chance to try the combination but if you are prepared to sacrifice AF speed for reach you can stick a 1.4xTC on and the IQ is still very good indeed. Effectively that is 400 x 1.4 x 1.6 = 896mm which you can crop still further in PP.
As @xelas has pointed out already though, the smaller the aperture the less light and the only alternative is to bump the ISO or decrease the shutter speed. On a sunny day not a problem hand holding , on a dull day you need to steady the set up with a tripod or bean bag otherwise the slightest movement is exaggerated the more you magnify the pixels.
Overall though, when it comes down to price the Tamron appears to be at least half the price of the Canon if not 40%, a not inconsiderable amount! So how come it's cheaper ? Has to be build quality I would have thought, it weighs 500g less than the highly rated Sigma Sport ( 2860gms) equivalent so the weight saving must come from materials. For easy hand holding (1570 vs 2010gms ), consistent results and overall quality the 100-400 Mk2 wins but it is a lot dearer.
Another benefit of the Canon lens is the minimum focus distance of 0.98m vs 2.70m for the Tamron. That makes it a pretty good macro lens if you want to dabble without buying a specialist lens so you can consider that a saving!
In the longer term I think the Canon is a better bet as it's a proven winner, highly rated and will hold it's value better.