So overall, the IQ is much better than the Tamron 17-50mm f2.8 VC? How is the AF compared to the 17-50mm?
Check this forum thread.
Over than 1000 post here. Thus, I believe the reviews do not bias Canon or Tamron. Overall the lens has better IQ than Canon 24-70 L. The VC is amazing. And the price is much cheaper than Canon 24-70mm. It's almost perfect except the same AF problem with Canon 24-70mm. About 10 to 20% of them may need to do MA.
Forgot to answer your another question. Based on the posts on the thread, the AF of Tamron 24-70 is very fast. It's noticeable slower than Canon 24-70mm, but you won't feel slow.