AFAIK, there is no real difference between the demo and the full retail version.
It really is a demo of the full version and not a beta of any kind so it should be fine, but it's worth setting it to medium and then playing around with other settings to see if it makes it look better but keeping the playability.
In the console (tilde button) you can enter commands that tell you how the game is doing with your gfx settings.
Console.showfps 1/0 (When enabled shows your current Frames Per Second (FPS) in the top left corner of the screen)
Console.showstats 1/0 (When enabled shows a range of system performance information)