I would be a bit worried about the size of this new 16-50L IS.
I think the most notable advantage of the 17-40 is its compact size. However, if the evolution would involve a change similar to 16-35L I to 16-35 LII... (and don't forget how IS tends to make the lens bigger)
Can you show us a lens that is bigger due to IS? Not one that I can think of - I could be wrong on this.
I also don't believe that 1 mm at front end and 10 at long end will greatly increase the size of the lens. it would be smaller than the 17-55 2.8 so it can't be too big...
How can putting on additional components not affect the size and weight?
70-200 f/2.8L USM 84.6mm x 193.6mm, 1310g
70-200 f/2.8L IS USM 86.2mm x 197mm, 1470g
(*stats from Canon USA website)
I haven't compared the two side-by-side, but any extra weight accounts for a big difference at the end of the day (as for whether you can tell the difference of a several mm in dimension, that's subjective).
However, if 16-50 f/4 L was indeed to come, my bet is there'll be notable differences, since its both adding IS and focal length. We'd be lucky if Canon keeps the filter size at 77 *shrug*