Never done that comparison as I haven't had access to the right equipment, but interesting idea. That said, BBC HD looks better than a lot of other HD channels on Sky HD anyway, so I suspect any downsampling to be minor. It's like buying a new TV, you can go into the shop and compare them all side by side and see which one is best, but when you get it home you're only direct comparison is the TV you had before and it will always look better than that one.