The 811 outputs 480I on the composite and s-video jacks. There is no way to adjust the SD at all. That said, when you are in HD mode, you can set your 811 for 480P, 720P, and 1080I. Whatever you set it for will be what is outputted. The 811 will not output whatever signal it sees out as that signal. An example, ABC transmits HD at 720P and CBS is at 1080I. You have to set the outputs manually to accomodate each. Even that doesn't matter much, because you should set your 811 output to that which is native to your TV. There are not many that will have 720P and 1080I as native resolutions.
On the other board, some were saying that their SD programming looked better, while in the HD mode, when they changed the output to 480P. I experimented with this on my Mits 65". I found that there was little difference between the 480P in HD mode and 480I in SD mode using S-video. My Mits has a pretty good line doubler, that is the difference.