Extremely useful article and analysis. Thanks. This will make good reference material for the future.
I was also surprised that you get 95% of the performance with x4 until I remembered that x4 v2.0 is the same as x8 v1.x...But good to know, and also good to know that the figures still hold true even on a top end i7 system.
(And that dual CPU server boards with "just" x8 connectors will still make good workstations)
Just to kick some dust, if an x4 is good enough (x8 v1.x) then so is AGP! Yes, PCIe is a better general scalable format, but, from a technical standpoint, AGP and it's x8 speed equivalence and lower latency would still be in the running if high end products were still available. This thought really surprised me!