This despite the fact that the human eye cannot actually resolve to finer detail than a 720p image at 7 feet viewing distance from a 40-50" television? Are you saying you have better than 20/20 vision?
Did you try doing this a/b blinded where you would not know which content was being played when? Have a friend switch the content on you without you knowing and see if you can tell the difference 75% of the time or better (you can probably get 50% right just by blind guessing...).
It is a 1344x768 plasma. It needs to slightly upscale 720 content. For 1080, obviously it needs to downscale. But, any upscaling (like from DTV or Blu-Ray) should be done at the source. Since my TV is capable of slightly more than 720, there should be a slight improvement with 1080 input - and there is. I don't have a way to a/b the ATV itself (HDMI/comp). At some point, I'll get a HDMI expander and make the issue moot.
To answer the "20/20" question: I used to have 20/15, corrected. (note "used to"

). I didn't do a scientific double-blind study (or much of anything that could be called a "study"). I've been in high-tech (engineering) AND graphic arts for way too long to not be able to discern subtle details. I can tell differences between which cameras were used in 1080 content (like "Planet Earth").
Total resolving power doesn't enter in to how the eye
perceives sharpness. Otherwise, (for instance) unsharp masking (re: Photoshop) wouldn't work very well. Sounds like you're referring to a thread which did all the math on how big a pixel will be at distance, vs. the eye's resolving power. The ability to discern one pixel from another doesn't take into account how the brain perceives detail - it's much more complicated than the simple math. Think about why a glossy screen looks crisper than a matte one. Both have the same size pixels. The glossy enhances contrast - making the image appear sharper. It's somewhat like what unsharp-mask does, which is to increase contrast near edges.
The OP was asking for a real-world comparison between HDMI and component. Since I have a similar mixed setup, I had some experience to share. I will agree, the best, most trouble-free method, would be a direct digital connection. But, with this particular combination of TV, cabling, and ATV, there is apparently little, if any, difference. I'm sure there are component capable TVs with less than adequate a/d converters, filtering, DSP, etc, whereby there would be more differences between the two. That's not the case here.
When it gets down to it, if the OP tries component and thinks there's a difference, then there is. In my case, there's nothing I can see that's making me go out and buy a HDMI expander.
I would actually like to put everything on HDMI, just to help clean up cabling, if nothing else. But, the expanders are still a bit pricey. I also need more optical audio inputs, so I'm looking for a combo expander.