Soon as the sun sets or you go inside, that extra stop makes all of the difference in the world. The 17-40 is a great lens, though. I had one and sold it after getting the 16-35mm. And I can guarantee if Canon or Nikon came out with a 2.0 or 1.8 16-35 at three or even four times the price, there's be plenty of people in line to buy it since even 2.8 can come up short.
Canon has the 24 1.4, and the 35 1.4 for low light work. Both awesome lenses. a 16-35 2.0 would be enormous and heavy. I'd pass on that zoom and go for the primes anyway, and they'd still have a full stop advantage.