Hi again!
To be honest, I did not expect so many replies. Thank you all for your input. As I see there are many different opinions and excitement, but still no clear answer. At least I still don’t get it. I shoot different objects with Canon EOS 600D at about 100-300 meters (328-980 feet). With EF-S 18-200mm f/3.5-5.6 IS lens I get quite satisfying results for my purpose. Only thing I’m missing is a little extra zoom. While planning to get a new lens I consider getting a higher quality one.

I think to give a clear answer to my question a real-life test is needed. Unfortunately I don’t have an opportunity to borrow such lenses from anyone and our stores are not going to allow me to do this either. Maybe someone has those lenses or can borrow them and test them in a distance.

So the test would have a camera with APS-C sized camera. For this test I believe there won’t be a big difference which model it is. Take 3 photos at a distance of approximately 300 meters (980 feet), but definitely all there at the same distance with the same camera:
1) one photo with EF-S 18-200mm f/3.5-5.6 IS Lens (or similar EF-S lens) at 200mm
2) one photo with EF 70-200mm f/4.0 L IS USM Lens (or similar EF lens) at 200mm
3) and one with EF 200mm f/2.8 L II USM Lens (or similar EF lens) which has fixed 200mm
4) unfortunately there is no EF-S lens with fixed 200mm, that I would know of.

Then if those photos are uploaded to this forum, we all can compare them and see the difference. Of course if such a test will take place then it could be expanded to more different focal lengths, object distances and different lenses. What you think? Would it make few things clear?

Anxiously waiting for results, new thoughts and ideas