im not even arguing, but the fact is in the past 2nd from high end parts have almost always been clearly faster than the previous top of the line.
http://www.techreport.com/reviews/2005q3/geforce-7800gt/index.x?pg=5
http://www.techreport.com/reviews/2005q3/geforce-7800gt/index.x?pg=7
7800GT vs. X850XT PE
I could go on and on with examples that go even way back further into the past, but there's no such thing as a "fact" as you're trying to present it.
Once more shader intensive applications arrive in the immediate future D3D10 accelerators will widen their current gap to current D3D9.0 GPUs even further and in that regard not even ATI will be able to perform any wonders with ~$400 R600 variants compared to their own high end predecessors.