From my experience Serious Sam 3 gets good crossfire scaling indoors, but outdoors (which is like most of the game) is still crappy. This is a confirmed critical bug by CroTeam. Bare in mind that my tests are based in Level 2 (The Spider's Nest which is mostly an indoor's level).
You score ~30 FPS with a single GPU, this rings me a bell that 99% you experience CPU bottleneck !! Please share your CPU model and what's your SS3 settings and resolution ?
Judging from your screenshots, CrossFireX is a bad feature for your system playing SS3. Two thinks come in my mind:
1) This is most likely due to your CPU bottlenecking your performance. Meaning that your CPU is not going to allow as much performance as your GPUs allow. That's why I had to overclocked my Core i5 2500K up to 4.5GHz. Crossfire-wise, CPU bottleneck can decrease performance in some cases when it doesn't have the CPU muscle to feed it. Unigine heaven has almost no CPU stress, so a slow CPU doesn't show up in there and that's why you saw such a performance boost in DirectX11.
2) Possibly poor performance from a current driver. Try running the game in Windows7, but don't forget to install the latest CAP profiles. Without those, crossfire doesn't perform as well.
Generally speakin, AMD have serious issues with their drivers both in Windows and Linux. Once in a while they fix & patch things and then they release CAPs to improve performance over some hot game titles (such as Skyrim, Starcraft 2 etc). That has been told, it's quite possible that AMD Linux drivers in conjunction with Steam beta and its OpenGL-to-DirectX porting system, to behave strangely. Unfortunately, I have already shipped my 6970 back to AMD, thus I can't bench in Crossfire any more. However, any time soon, I will try to find a couple HD 7000 series in order to play with Serious Sam 3 and Team Fortress,
Just for you information, phoronix has published some Crossfire benchmarks back in 2008. In QuakeWars, there is a huge scalling and crossfire works great. But in Quake 4, there is no Crossfire boost. Now supposing that this article was written back in 2008, we have 4 years of improving drivers. Do you really believe that AMD guys are so dumb that they managed to ruin Crossfire performance that bad as you imply in your in-game screenshots ? I mean, you don't even get equal performance, but worse. If I were you I would probably do some more testing with proper hardware and then fire accuses. Anyway, no hard feelings.
my 2 cents
PS: I agree with you for Unigine Heaven, no Crossfire profile for OpenGL both in WIndows and Linux.