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I've seen some who have been very positive about their experience with the DO and a 1.4x and others who have been disappointed with the results.
After having used a 400 DO for about 6 years, I've come to the conclusion that results with the 1.4x are very dependent on how well the particular copy of the DO and the copy of the 1.4x are matched. If they are both very close to spec or if the extent that one is a bit off is balanced off by the other being off to a similar minor extent in the opposite direction, the results can be quite good. On the other hand, if either is off by a good bit or if the variation from spec of one builds on the variation from spec of the other, the results can be less than satisfactory.
Over the years, I've used mine with 5 different 1.4x's. One was a Tamron SP 1.4x, 3 were Canon 1.4x II extenders, and 1 was a Canon 1.4x III. My results with 3 of those extenders have been excellent, and I would describe the minor level of image degradation as being on about a par with what I see when using my 500/4 with a 1.4x. Results with one of those extenders were okay but not great, and the results with the other were not good at all. Curiously, the excellent results have been with the Tamron SP 1.4x, one of the Canon 1.4x II extenders, and the Canon 1.4x III. The results with one of the Canon 1.4x II extenders were okay, and the extender that produced the poor results was the other Canon 1.4x II.
The shot below was taken with the Canon 1.4x III and DO mounted on a 7D.