I see little reason to spend an twice the cost.
I have had no problems using the 17-40 4, add in that it is slightly smaller and wider and I am set.
I am happy I decided to get it rather then save up another $700 for the 2.8. At that wide I really don't need the 2.8 anyway. maybe @ a concert or something, but I rarely shoot those. The 4 is plenty fast for me.