Hydralisk_X,
I wonder if you're hitting the CPU limit. Here are my findings on scaling with the Eyefinity6 card.
http://www.widescreengamingforum.com/wiki/ATI_Radeon_5870_Eyefinity6_-_Featured_Review_-_Page_15
Do you have Just Cause 2? The only scaling benches I've seen are from Tom's where it appeared that from 1920x1200 to 2560x1600, the scaling suffered a huge loss. But I haven't seen any triple screen benches yet. Personally it's the one game that bothers me a lot since I only see about 20-25% scaling and was one of the reasons I got a 2nd 5870. http://www.tomshardware.com/reviews/geforce-gtx-480-3-way-sli-crossfire,2622-10.html
For the other games, the scaling isn't horrible compared to single screen, but they're most definitely worse going from 1920x1080 to 5760x1080. Do you think my i7 860 @ 3.4 ghz is bottlenecking at such an extreme resolution? I noticed in your test setup you have a i7 920 @ 2.67.
I did browse through the review and I noticed that in general the single screen resolution scaling was better than triple screen except for far cry 2. It's hard to get hard numbers from the graphs since I have to interpolate, so I just eyeballed it and tried to guestimate.
Overall, the scaling isn't bad in triple screen although I'd like it to be better and get closer to single screen scaling, but Just Cause 2's terrible scaling really bothers me.