This test was taken with the following lenses:
+ Canon 100-400mm f4.5-5.6L with the cheap Kenko 1.4x Teleconverter (with Hoya HMC Super UV filter attached)
+ Canon 100-400mm f4.5-5.6L by itself (with Hoya HMC Super UV filter attached)
+ Canon 28-105mm f3.5-4.5
+ Canon 50mm f1.8 mkII

All pictures were framed approximately the the same, and the distance was changed as the focal length changed. Pictures taken in RAW and converted to JPEG at level 11 quality with NO sharpening applied.

Taken with IS OFF and tripod mounted with Mirror lock-up. Remote shutter used.

Each 100% crop is approximately 170x240 pixels.