I currently own a 70-200 f/4 L USM (non-IS lens). I bought it when I shot crop. When I bought a full frame camera I struggled adapting to the focal range, but have grown to like it a lot.

I'm considering upgrading my lens to the f/4 IS version for improved images through stabilization and the new optics and a more stable viewfinder. Primarily uses include general walk around, landscape, and the occasional portrait or sporting event.

I don't have any interest in the f/2.8 variants due to cost/weight. I could consider the 70-300 L, but like the 70-200 f/4 form factor and appreciate the reduced weight.

I've researched the decision fairly in depth. I have questions for anyone who can help.

1) The 70-200 f/4 L IS is well documented in its early days of release as having greatly reduced image quality @ 200mm from distances of 6 feet on down to MFD. Do you find this to be true?
2) It appears image quality for the IS lens versus non-IS is slightly improved. The site's tool as well as two other well respected sites each show this to be the case. Some users don't seem to notice the difference in actual use. Do you? I typically don't print larger than 11x14, but I have to admit that super sharp images on my monitor bring a smile to my face.
3) I own a EF 1.4x II teleconverter that I use occasionally with my lens. Teleconverter performance appears to be a push. Any comments?

Thanks! The cost is a fair amount of change for me -- any help is appreciated!

Some of my work with the 70-200:

2014_10_18_9132_LR_upd_16x9 by dthrog00, on Flickr

2014_07_03_7377_LR_upd_8x10 by dthrog00, on Flickr

2013_09_22_4006_upd by dthrog00, on Flickr

2014_04_19_6946_upd_16x9 by dthrog00, on Flickr

Regards
Dave