It doesn't look like overexposure to me. Are you sure you are using the default single rather than multi point alignment in Registax? Multipoint alignment can sometimes cause that weird post modernist effect.
I would use the barlow, 10fps, 1/25 shutter, leave brightness on 50% and gamma about 20%, saturation near 100%, and then get the correct exposure by adjusting gain only. Reduce gain from full until there are no saturated patches on screen. Examples from a C11 with similar settings here:
http://www.pbase.com/rigel003/jupiter_images
Good luck.