Doubt this will show very well but I've just been doing some tests with my 1.4x TC and comparing it against using the lens without TC and performing a 1.4x enlargement using Topaz Gigapixel, and then using Topaz sharpen AI to get the best possible results. All shot on a tripod using remote shutter, 100% crops
1. Gigapixel enlargement left vs 1.4 TC right, gigapixel looks sharper
TC right, Gigapixel Left by
Toby Gunnee, on Flickr
2. Gigapixel enlargement left vs TC right then sharpened in Topaz, TC image looks sharper
TC and Sharpen AI right, Gigapixel left by
Toby Gunnee, on Flickr
3. Gigapixel enlargement left vs 1.4 TC right, gigapixel looks sharper
Repsol TC right, gigapixel left by
Toby Gunnee, on Flickr
4. Gigapixel enlargement left vs TC right then sharpened in Topaz, TC image looks sharper
Repsol TC and Sharpen AI right, Gigapixel left by
Toby Gunnee, on Flickr
5. Gigapixel enlargement and sharpened left vs TC right then sharpened in Topaz, pretty much identical
Repsol TC and Sharpen AI right, Gigapixel and Sharpen AI left by
Toby Gunnee, on Flickr
Here's a link to the screenshots if you want to see it better
www.dropbox.com