The HD programs seem as clear using the component as it did with the HDMI cable.
I hooked up both HDMI and component and then switched using the TV. The image looked better with HDMI so that is what I use. You may want to do your own test.
It makes sense that HDMI looks better then component. Especially, if your cable system is all digital (like mine.) To go by component it has to convert from digital to analog (component) and then back to digital. Any conversion will cause a reduction in image quality. Also analog (component) can introduce artifacts from the cable.
It seems you can get an upscale picture using component, if the equipment is designed for it.
Only DVD players can't upscale using component due to licensing restrictions. Other devices, such as a cable box, can upscale.
One thing you mentioned is that your cable box upscales, but so does your TV. And it probably upscales better then your cable box. The normal advise is to have your cable box pass thru the signal and let your TV upscale.
The reason for using upscaling DVD players, even through the TV will upscale, is that DVD players typically do a much better job of upscaling then your TV.
So most devices today (DVD, Cable, Audio/Video Amplifier, TV) will convert the signal. The question is which one will do a better job. The typical order from worse to best is Cable, TV, DVD. Audio/Video Amplifiers are all over the place. But, your mileage may very so you have to do your own testing.
David