Given Canon's obsession for pixel count over sensitivity, the potential stop gained by f/2.8 compared to f/4 zooms would seem to be countered by the extra ISO step needed for the same noise level compared to a FF unit.
Exactly. Few people get this - especially those who make statements like, "I'm not upgrading to FF until they make a FF equivalent of the 17-55mm f/2.8 IS," in reference to the magical unicorn also known as a 24-70mm f/2.8L IS. In fact, the 24-105mm f/4L IS when used on FF specs out better than the 17-55mm f/2.8 IS on a crop body. The FF-equivalent of the latter is 27-88mm f/4.5 IS, meaning the 24-105mm is wider, longer, and faster. It's true that f/2.8 still yields a faster shutter speed than f/4, since the crop factor doesn't affect exposure (apparent effect is on DoF only), but as you point out - FF has a 1.3-stop advantage in ISO noise, meaning you can bump the shutter speed up a stop by using a higher ISO on FF to match the shutter speed you'd get with f/2.8 on a crop body.
Correct.
But here's another way to look at this. Suppose we want to emulate 17-55/2.8 IS on a FF body. We'll need a 27-88/4.5 IS lens to emulate field-of-view and dof, and we'll crank up the iso by one stop to emulate shutter speed.
So: the 17-55/2.8 IS can do almost the same as a 24-105/4 IS on full-frame. Not quite the same focal length range, just a tad shallower dof, and even with iso up one stop there's still a slight noise advantage. But it's close.
And that's pretty impressive if you think about it, for a sensor that's 2.5x smaller.
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