Could you take an identical picture with both lenses, and see if we can tell the difference? No editing, though. I wonder if it's noticeably better.
Between a kit 18-55mm plastic cased lens and a 70-200mm F2.8? Bokeh for one will be noticeably creamier with the F2.8 and the sharpness advantage will also be a bit obvious i think, i've shot with both, and i think I'd be able to pick out the 70-200.