Don't bother trying different HDMI cables for that length - they are not the problem, as you are getting the video data delivered successfully.

I would not see it as a flaw (serious or otherwise) if the display needs some different settings between a component video input and an HDMI input. For one thing, many newer displays store separate settings for different inputs. Mine doesn't offer this, and while it works out OK there are some differences in settings - I calibrate the DVI for my highest quality sources and count myself lucky that the HDTV broadcasts work reasonably well with it. If your display does offer this, the HDMI input may not be calibrated at all even though you adjusted the component input using your old Onkyo. Even if it doesn't, though, you have two different video signal paths (one with an A/D conversion that you have calibrated for and one without that conversion that you haven't calibrated for), and just like with analog vs. digital audio that may be affecting the output. I'd recommend re-visiting your calibration with the 980H and the HDMI connection together before making any final judgment. Otherwise, you may very well have something out of adjustment, which would very easily produce the findings you've reported, and a little time spent calibrating the video could give you a better picture than either input type is presently providing.