I dunno ... you tell me? When it came out I had heavily OC'd 5850's in xfire, and it was totally unplayable maxed out at 1080p. I had to run it on DX11 'High' w/OUT ADoF or 4xMSAA in order to make it 'perfectly smooth' from start to finish. And that was TWO 5850's my friend.
There will be parts of that game you'd find run okay, but there will be MANY parts of that game where it would bog down into the low teens in FPS.
I'm sorry, but you did NOT play that game start-to-finish 'maxed out' on a 6870 at 1080p, 'perfectly smooth'. Unless you simply don't perceive it when your game is massively lagging.
Otherwise, there is no way
You wanna prove me wrong, there's a Metro benchmark out there ... I'll be waiting ...
EDIT: And just because I'm a kinda guy, I did a run on a single 470 at 800/1800 (which is faster at Metro than your 6870 will ever be), MAXED OUT at 1080p.
FPS averages just over 21, barely breaks 30, and dips down to under 9 (nine) FPS at one point. VRAM usage in the OSD showed 1100-1200MB the whole time.
Now are you really going to sit there and tell us that your 6870 1GB can play this game at these settings 'perfectly smooth'?