It is difficult to quantify because the new screens are based on a transflective technology that allows sunlight to pass through the LCD and reflect back out, thus the screen becomes brighter in bright environments. I've been telling folks "about 50% brighter" as a rule of thumb...but the actual readability, IMO, is many times better in bright sunlight.
I invite some of our beta testers to chime in here and share their experience with the new screen.
Thanks!
Walter