This guide will show how to enable the motion blur effect independent of which shader settings (Low/Medium) you have assigned:
1.Go to Library
2.Right click on Portal 2
3.Select "Properties"
4.Select "SET LAUNCH OPTIONS..."
5.Paste this command: +mat_motion_blur_enabled 1

DONE!!! That's it! (also works in Left 4 Dead 2)
Now you can use the terrible motion blur effect in any shader setting!

Note: You have to paste the command directly into the launch options, it won't work if you type it in the ingame console!

Want to disable it? Click here: http://steamproxy.net/sharedfiles/filedetails/?id=1103615951