I am also running 3240 x 1920 eyefinity portrait mode. Yesterday I manually set the resolution with an autoexec.cfg file and played fine for 2 hours however the game does not run very smoothly. My FPS constantly stays below 30 and when it hits the low 20s it starts to become a disadvantage. You can't use a shotgun when your FPS is that low because it is too hard to aim. I don't think that my PC should struggle playing this game. Here are my specs:
win7 ult x64
amd 6core 1090t
6 gigs of gskill 1600 ddr3 (my friend is borrowing my 4th 2 gig dimm for some troubleshooting with his PC but I usually have 8gigs)
2 sapphire 6970 in xfire
raid 0 300gig raptors
Now you are probably wondering what graphic settings I had the game on. When I first opened the game up, I naturally went to the graphics settings and bumped everything to the max like I do with most games (except no AA). With high textures and shadows, post processing, ambient occlusion and soft particles on my frames were below 20. I dropped textures and shadows to medium and disabled the other goodies and my frames are still below 30. This is where I have left them for now.
I installed 11.5a hotfix yesterday before playing and I noticed that there isn't a new xfire app profile out yet to handle brink. I tried disabling crossfire (because some games don't like it) so that I could enable vsync to see if that would help. My frames actually jumped up to the 40s if I was standing at the spawn point with no other players around but it dipped back below 30 as soon as I entered battle. I don't really want to have to disable xfire to play this game but I don't want to play at a constant disadvantage running 22 FPS either. I get better FPS than this in metro2033

I guess all I can do is hope for a patch? but for what?