Wouldnt it be nice just to have a standard chart we can all print out, set up at X distance with X standard settings and compare our image with a known best image.
Why is it so difficult.
Reikan FoCal does something along those lines with their aperture sharpness test (everyone is using the same target, and the software determines the settings so they are standardized), with the aggregated results reported online. But the idea is complicated by the fact that setups will vary - particularly, many people shoot test targets without bright, even
illumination. Also, where would the 'known best image' come from? Reviewers generally only test one, sometimes 2-3 copies. Roger Cicala (lensrentals) tests many copies of some lenses, there's often quite a bit of variation.
I suppose the bottom line is that if you buy a lens, and you're not happy with the resulting images, you should exchange it or send it to Canon. BTW, Canon does have reference bodies and lenses, so they can determine if your lens isn't up to spec, and correct it if that's the case. You might want to send your 70-200 II in (and if it's still under warranty, it'll be free).