Dave:

First off, nice shots!

I have the 70-200 f4 IS and am very happy with it. I've never owned the non-is version though, so I can't make direct comparisons. I do find the IS version very sharp and have never noticed a decrease in sharpness close to MFD. I tend to use it for outdoor shots of my toddler and try to fill the frame, so a lot of shots are close to mfd.

I also use the 1.4 II teleconverter with this lens (probably less now that I have the new 100-400 ) and find the results good. Sharpness does decrease, but definitely still acceptable. Here are a couple taken with the 70-200 and 1.4II:

Maui_180 by Stephen, on Flickr

Coyote_05 by Stephen, on Flickr

Some other similar shots on Flickr with this lens combo as well as some with the 70-200 without the tele.

Hope this helps.
Stephen