I can't give you a technical answer or reason but I upconvert my Sky+ signal through my A1SR and on an 8ft screen via a Sharp 10k I am always blown away by the image quality. Every single movie channel looks like DVD quality and all other channels are not far behind. Even 24 the other night looked amazing - now that's saying something
I have absolutely no complaints and when I used a RGB>Component converter I could see no visible improvement.
Works for me