Whoa Whoa Whoa. Something is very wrong with your 4890 benches! You have the 4890 listed as 20 FPS avg at high settings at 1920X1200.
This has to be very wrong, I'm running a 1GB 4890, at 1980X1020. It does have a factory overclock to 950 mhz, but that shouldn't make a huge difference as I believe default 4890 clock was 850. Anyways, I'm playing the campaign at all ULTRA settings, mind you, NOT high, with the exception of terrain quality locked to medium (as with all DX 10 cards), and SSAO instead of HBAO, no MSAA, FXAA set to HIGH, motion blur on, 16X AF (settings because I heard HBAO and MSAA are performance killers).
And I'm getting, in the early parts of the campaign, oh I would say a solid 30 FPS. Very playable. Sometimes in less demanding areas I even sometimes see the FPS shoot up to 45-50, though it often dips to the mid-upper 20's as well in firefights. I havent run a fraps average, but my guess is it would be averaging perhaps 35 FPS, and no lower than 30. That's fully playable for me, I find 30 FPS to be fine (and I console game a lot too where 30 is the norm).
And the rest of my rig is weak too, it's a Q6600@2.8, 4GB of DDR2.
So yeah, 20 FPS on lower settings than I'm using, something is wrong. By your chart 4890 is unplayable at high settings+1080P, yet I'm playing it very nicely at mostly ultra 1080p, forget high!
It could be something to do with the level, as I said I'm early and have not reached the "Go hunting" level yet. Still, I'm doubting it's so much more demanding than the rest of the game.
Another clue is that the 6850 should have similar grunt to my 4890. My 4890 is 800 shaders @950, a 6850 is 960 shaders @775. That's almost exactly equal in power when I did the math (I know shaders arent everything but it should give us an idea). So for 6850 to be averaging 40 and 4890 averaging 20 again is a red flag. And as I showed, I would guess my 4890 is averaging maybe 35, which would put it close to 6850 where it belongs.
So yeah, imo your 4890 benchmarks are wrong, I'm playing at 1080P mostly ULTRA at 30-35 FPS with a 4890.
I did notice you have HBAO on as well where I'm using SSAO (then again I have multiple settings at ultra versus your high!), but again I'm not sure that would kill performance that much.
I'll revisit this comments section in a day or two, double check everything, get a fraps avg to be exact, maybe see if HBAO makes the difference, but as of now must disagree with your 4890 bench.