I'm surprised your finding that resolution to be an issue. I have the 5850 running 5760x1200 (on a system much slower than yours) and the raw speed is quite amazing. Eyefinity has its glitches, but for me, horsepower hasn't really been one I could complain about.
. Well, what games are you running?
I'd like to see what your in-game settings are so we could compare apples-to-apples.
Try Crysis Warhead with everything on Enthusiast if you want a lovely slideshow. Dirt 2 can't handle any antialiasing with settings above Medium (horrible stuttering as you overwhelm the framebuffer apparently). World in Conflict DX10 also stutters pretty badly if you try 4x MSAA and set everything to High.
I'm not surprised really. A single 5870 struggles to give 60fps on these games at 1920x1200 with everything maxed out. At 5040x1050 it's just overwhelmed.
What worried me the most is that you really need 2gb for framebuffer at these resolutions. Once you start turning up the AA, that 1gb gets used up quickly.