100-400 used to be my standard combination usually with 1.4x Canon Extender. I would say that the quality of picture does not suffer much if you have reasonable light. Even better if you can stop down a bit. My experiences with 2.0x Extender and this lens are not very good. Very few lenses work well with a 2.0 converter and this is border line. Obvious problem is that it only works in MF with any camera as 1.4X works quite in with AF if you tape the last three pins to prevent it from reporting to camera. This way 400D does AF reasonably, 40D not very well. 1D series bodies work as AF up to 8.0. If you want to see results with especially 100-400 and converters please check the greatest authority of long lens bird photography Romy Ocon http://www.pbase.com/liquidstone/root. He also regularly comments as liquidstone at DPreview forum.