Maybe not double, but it should be nudging 10Mbps at least, I should think. I was nudging 650K down before, so it certainly shouldn't be less than that...
Don't quite understand. You want overscan on when it's showing SD material, and Off when it's HD? Do you mean you want the TV to do this itself? Or do you mean the option is bloked when viewing SD or something? I persoanlly know of no TV that will disable and re-enable overscan at will. No a major expert here, but surely it's either on or off, at your discretion? Auto might be the only way otherwise the TV only 'sees' a 1080i signal whether it's SD or HD. As the box will be saying that's what the signal is.
On the other point, my Sony TV allows me to choose completely different settings for each input, whether they are HDMI or not, but I can choose to have 1 HDMI with overscan and 1 without if I so choose (which I don't, looks better without overscan I reckon).