Ok then, do a graph that shows how half of 5870's core (in this case, a 4890) still benefits from having 5870's full bandwidth all to its own.
I already provided sources with several benchmark tests (dozens, in fact). Those numbers are not from my own ass, so you'd be able to touch them with a 10-foot pole! Hehe...
I mean, you are basing your convictions on just one test. That other theoretical fillrate test with the memory downclocked is such an incomplete test that if you tried downclocking the memory much further, like 50% or more, you'd still be getting the same results, therefore it would have wreaked havoc with the graph that you just created. It'd make your graph look like fireworks that have gone haywire!
The red line in the graph, once it reaches 1300MHz, might already be in the error-correcting slow-down territory. It could also be plagued with more than one step of latency increase when overclocking the memory. That's why it's better to not be too absolute in little-developed convictions.
I'm basing mine on the trend of a 4890 and a 5770 and a 5870, and also 4890's/5770's in Crossfire. It's just that two halves of a 5870 does not scale as badly as you would think with diminishing returns that are far worse than Crossfire scaling with two 5770's. A 5870 core is still a massively multi-core processor that needs to be fed gobs and gobs of bandwidth in order to function.
I know that the analogy of 512-bit to 1GB is not exactly the same thing, but in some scenarios, there is a HUGE benefit from having twice the buffer. The same goes for bandwidth.
If you truly think it should be such a linear world, please try to think about the effects of reduced bandwidth. It does not reduce performance linearly, even if you cut the bandwidth in half. Most games do not give linear predictions. One game would need to be fed 200Gb/s of bandwidth fast enough if running at 1920x1200 with 4xAA to show a large boost in performance compared to 190Gb/s. Enabling Adaptive AA can take up so much more bandwidth especially if there are lots of alpha textures. Another game would do just fine with simple polygonal fillrate and simple AA that are handled by the shaders, and only experience linearity in relation to the speed of the TMU's/shaders without needing gobs of bandwidth.
Sometimes, overclocking the memory by 5% would give a 10-15% boost in a certain scenario. I'll have to find a few of them, but after looking at millions of benchmarks for more than a decade, yes, I'll have to backtrack through a lot, heh!
It's like having a construction worker who just turned into an 8-foot giant, but now he has only one arm to do all of the work. Do you ignore that missing arm and say that it's all because of the drivers program in his brain that is not quite-so-efficient? Me, I'd just point the obvious and say that he'd do whole of a heck lots better with two arms, dammit!