That Mass Effect almost has a tunnel vision thing going on? I noticed this on both Xbox and PC. What I mean is the corners of the image are darker compared to the center. Is it just me?
On the Xbox 360 version, this filter is optional (it's called "cinematic" IIRC) , but the game looks really flat without it. The thing is the filter includes more than just vignetting; it also adds anamorphic lens flare etc. I think the game's visuals were basically designed with these effects on.
You could try setting all instances of the bUsePostProcessEffects variable to false in the BioEngine.ini file. I think it will rid the game of the vignetting and other effects.