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I've seen a wide range of reports from 400 DO users with regard to the performance of the lens with a 1.4x. Some have been quite pleased with the results, and others have not.
Over the years since I bought my 400 DO, I've tried using it with 4 different 1.4x converters. Two were Canon 1.4x II converters. One was a Canon 1.4x III, and one was a Tamron SP 1.4x. With one of the Canon 1.4x II converters, loss of image quality was significant. With the other Canon 1.4x II, with the Canon 1.4x III, and with the Tamron SP 1.4x, loss of image quality was minor and somewhere in the same range that I've come to expect from using a 1.4x with my 500/4. Further, the hit on focus speed with the 1.4x is relatively small provided that the lens' focus limiter switch is set to shorten the focus range.
Thus, my conclusion is that variation in image quality of the 400 DO with a 1.4x is likely the result of differences in how well the particular copy of the 400 DO matches up with the particular copy of the 1.4x. I would also guess that a carefully done micro adjustment can minimize any loss in image quality.