I don't think there will be that much of a difference in terms of DOF between 2.8 @ 200mm and 2.0 @ 200mm. Have a look at the comparison pictures of the girl in this review : http://www.the-digital-picture.com …L-IS-USM-Lens-Review.aspx.
here's the thing: there is a clear difference in background blur between F2 vs F2.8, that you can even see them in web sized images. You're saying that it's not much different, YET at the same time, you'de have to view 100% crops to see the sharpness differences.... if you can even spot them.
I will post some pictures tonight when I go home and compare my 85L @ 2.0, 2.8 and 70-200 @ 200 2.8 and show you guys what I'm talking about in terms of IQ.
this will help greatly, and I'm sure the gearheads around here will pick up on it.