CrossFireX support was not initially available for Eyefinity users. That was added in Catalyst 10.2, along with the new driver structure that allowed for easily updatable "CrossFireX Application Profiles." Since that time, ATI has improved the stability and performance of CFX for Eyefinity with updated drivers and improved application profiles.
CrossFireX is basically overkill for a widescreen user, but is a valuable tool for Eyefinity users. Widescreen CFX offers 60% - 80% improvement, but your initial framerates are probably well past 60fps. Eyefinity CFX offers 50% - 70% improvement, and is beneficial in many instances.
Until ATI comes out with new hardware, or gets Tri-SLI to work with Eyefinity, I'm basically done benchmarking. The one caveat will be benchmarking in 5x1-Portrait, but that will only be on the Eyefinity6 and E6 in CFX. I'm working with NVIDIA to get some Fermi cards to test in 3DVS. I'm really excited about competition in the multi-monitor space, and want to put some numbers to the NV offerings.
If I can get World in Conflict working properly, I will run some tests on that. I wouldn't run the whole gamut of cards, instead focusing on some of the key SKUs. Same with LOTRO. I've got a rather automated horse run mapped out. I'd like to do it in DX9, DX10 and DX11.
With testing wrapped up on the 5870, 5850 and 5770 in CFX, I will be producing charts that compare performance to total price. This will be the backbone of our forthcoming Eyefinity Buyers Guide.
Looking at entire product line, it's difficult (if not completely impossible) to make simple recommendations. If you are solely a single widescreen gamer, I can't recommend a CFX configuration. The ATI Radeon HD 5800 and 5770 perform so well, it's simply not needed.
Eyefinity certainly benefits from CFX. The 5770 is hard sell if you're buying both cards at once. My recommendation would be to go with a single 5850, versus the 5770 in CFX. If you are stepping into Eyefinity on a budget, then staggering the purchase of a pair of 5770's is a viable option.
The 5800 series has similar performance from the 5870 E6 to the 5850. All of the cards perform admirably in a single configuration, but see significant performance boost in CFX.
For ultimate performance, the Eyefinity6 in CFX is the way to go (a "regular" 2GB 5870 is basically the same cost as the E6). In my single card reviews on the high end, I recommended the E6 due to it's options for expandability and the 2GB framebuffer. I still see the 2GB of VRAM as an advantage, and that plays out in the CFX performance between the original 1GB 5870 and the 2GB 5870 E6.
The 5850 and 5870 offer similar performance in many areas. You can save considerable money in looking at the 5850 in CFX, versus the 5870 in CFX - while maintaining a very healthy performance. While I haven't had the chance to test it, I believe the sweet spot for CFX will end up with the 2GB 5850 cards in CFX. The 2GB 5850 will offer the benefits of the additional frame buffer, without the additional cost of the six monitor support in the Eyefinity6.