If someone sees a "MASSIVE" difference between 576p and 720p (RDR), than surely they have the perception to see at least a very noticeable difference between 900p and 1080p.
You can't proclaim to have a technical eye for such disparity and then either downplay or mitigate the 40% difference in resolution at twice the pixels.
As mentioned above, I notice a huge difference between Halo 3 and Project Gotham 3, compared with Halo Reach and Project Gotham 4... and those weren't even as low as 576p. I do however struggle to differentiate between 1080p and 900p on my current setup.
The numbers are unimportant, because my TV set didn't grow in proportion to the resolution changes (in fact, it didn't grow at all), so the pixels simply got smaller to the point where I fail to discern them individually, until some horrid lack of AA sets them off crawling in motion.