Help check my lens quality

[Aratharu]

I'm having doubts about my newly-purchased 70-200 f2.8L IS lens... have a look at the comparison pics below and tell me whether this is "normal" or not.

The pics were taken with a 300D on Av setting, ISO 100, center point AF, no compensation, default Parameter, using a tripod, at 200mm zoom. First pic is at f2.8 and 1/3200, second is at f4.0 and 1/1600, third is at 2.8 and 1/3200, fourth is at 4.0 and 1/2000. The JPEGs are cropped only, no resizing nor any other post processing done.

Why the big difference in image quality between fully open and 3 stops up, for $1500 worth of lens? Opinions?