Yeah, I played with the Energy Saving mode quite a bit. I tried all the settings, and I can definitely see how it affects the brightness, but... (read on...)
It's not really the brightness that's the issue. Mirror's Edge is indeed a bright game, and every monitor/screen I've played it on faithfully keep it BRIGHT. (I've tried it on 4 different screens.) The problem with the A650 is the contrast and the artifacts that are created because of the contrast. Maybe "contrast" isn't even the right word (or maybe Samsung's quoted Game Mode contrast is incorrect?), but something happens in Game Mode that makes the edges look jagged (and even creates bad colors / artifacts). I should've taken a picture to explain what I meant, but unfortunately I didn't think to bring my camera. Here's my "artist's rendition" (using Photoshop) of what it looked like though:
The top picture is Mirror's Edge on three other TVs that I've tried it on. The bottom picture is Mirror's Edge on the A650. Contrast? Sharpness? Eye-hurting jagged edges? I'm not sure what the appropriate adjective is, but that's what Mirror's Edge looked like to me in Game Mode on the A650. Admittedly, this is a PHOTOSHOP APPROXIMATION, so the effect is inherently estimated/exaggerated. But it's honestly not that much of an exaggeration. It was pretty darn bad.
I personally found the lag to be too much in Movie mode and Standard mode for Mirror's Edge and MGS4, but I fully admit that there are probably games where it doesn't matter at all. And probably gamers for which it doesn't matter at all. Anyone who's shopping and concerned: Take your system into Best Buy and set it up on the TV and try it out. You'll be able to tell instantly if it bothers you or not. Unfortunately, there simply isn't a concrete answer here in the forums. Everyone has a different threshold/tolerance for lag.