I know some people here were waiting these years with me. I know besides myself, Spoonie, and a few others were also ragging in the forums trying to get some love for Surround/Eyefinity setups. I hope this is good news for some people.
Here are some screenshots with some details.
The login screen is centered
The options menu is good except for the accept/back buttons.
The Navigation menu is centered but leaves the bounds a little on either side. Undoubtedly it is 21:9 since that is the widest AR they support, and they actually added that last year.
The HUD is the only thing I've found at the edges of the outer monitors. I personally don't ever look at it since the game is so fast paced; however the mini-map is also on the left top corner. The in-game screen adjustment will bring it closer (Undoubtedly also to 21:9), but they end up in the middle of the other screen. The reticle is centered, and the correct scale. Also the PiP with Lotus and other people talking to you is to the left and slightly stretched.
The models, geometry, effects, and post processing is all correctly scaled and rendered. Including the elemental effects, weapon effects, lens flares, fog, and DoF etc..
The Codex, Mods, and all other stations on your ship are also centered menus are also centered but leave the bounds also.
The only one that is really stretched is the Equipment/Warframe station. It is however still totally usable.
The Hacking and other in game tasks/mechanics are all perfectly centered. (As far as I can tell anyway. I've yet to see one not centered, or unusable


