1.4x and 400DO

[stever]

i rented a 400DO last year and used it with 40D, subjectively kind of gave up on 1.4x

now testing with Imatest before packing it up for 3 week trip and finding
- lens performs marginally better than my 100-400 (a good copy afte some time and money spent) on 5D2
- with 1.4x it is no better than an equivalent crop without extender on the 5D2 - extenders don't seem to do well for me on the 5D2, but they've always been an improvement over the crop (extender require -15 focus adjustment)
- similar results with extender on 40D and 5D (40d results stink)

is this typical?  diffraction optics don't like extenders?

suggestions? (other than trading it for a 500 which is to big and heavy for my trip)

[DaveDn]

I use the 400mm DO with x1.4 on my 1DS3 and I don't think it is that bad. Obviously image quality will be reduced somewhat.

I don't see the image quality reduction any different than a non DO lens. I have done specitfic testing though.