The CX requires more setup, but the Opteka isn't doing you favors. It's a good lens, but it's also an adapted lens. Set up the camera to underexpose by -1. See if you like it better. The GP3 is a very good contender for the older AVCHD cams, however.
On a chart, the GoPro under exposes a bit. Sony is flat to slightly over. In reality, GoPro's shifting exposure can give you a few challenges due to the knee. Sony is a tad more forgiving. *generally* speaking, underexposing by -1 on the Sony cams that allow it, is a good practice, particularly if you'll be posting the footage.
i have mine properly set up with the highest quality settings, the 0.3 lens just looks like shit unless im an inch away from something and without it, the quality is ok but definitely not worth the hassle when i can throw my GP3 on my full face and get better quality. #frustrated
somewhere on here, I have a resolve chart from years back. IIRC, the Opteka resolved around 500 lines, or about half-HD, so it's a tad soft by comparison, but the price is very right on these. I use them for student jumps, because they wear out after a few months of daily use (I'm terrible about capping the lens each landing/takeoff).