Steve discovered a way to force a 2.2 Mbps stream on the Roku Netflix Player via Mark12547 on the Roku Forums.
There is an undocumented way to lock in a given bit stream.
Using the remote, hit Home Home Home Home Home Rewind Rewind Rewind FastForward FastForward (that's 5 Homes, 3 Rewinds, 2 FastForwards), keypresses about 1/2 second apart. You may have to try several times and the trick is to make the keypresses spaced out far enough. (Remember, the last two are <<< Rewind and Fast Forward >>>, which are on the bottom row of buttons, not the < and > keys to the side of the select button.) This will take you into debug mode. On this screen you would see something like this:
You can select the bit stream that you desire. The bit rates correspond to picture quality of 4 dots (2.2Mbps) down to 1 dot (0.5 Mbps).
As I said, the "debug mode" keystrokes are not documented, neither is this screen. That means it might change in the future and, if a microcode upgrade is loaded (which forces a reboot of the box), you may have to go back to this screen and set the mode again.
The good news is that the worst damage you can do on this screen can be undone by rebooting the box.