Robert, the left image looks a little out of focus... not sure if that was contributing to the result.
There are a lot of different steps in processing, and some of these (registax) rely on a good signal-to-noise ratio in your raw data. If the image is too dim, or too grainy due to lack of light or too much magnification then it can misbehave. Registax can not align properly if it can't clearly tell the difference between the features and the noise.
For every aperture there will be the "best" image scale, but it's worth remembering that Damian Peach was using the equivaent of a 4x barlow for his stunning Jupiter images last year from Barbados (or so I assume from his labeling them as "F/42") with a C9.25. You can see his images here:
http://www.damianpeach.com/best.htm
Your experiment might be more of a test of the limits of the camera than the scope??
regards, Bird