I'm not sure if it works exactly that way with Cypress.
Look at 5850 - OC it to the 5870 clock speeds and you lose like 3%, and you have the same rops but only 90% of the texture units and 90% of the shader power.
Look at 5970 vs 5850 CF - same story, 90% of the shader power, 90% of the texture units but pretty much the same performance.
Look at 5830 vs 4890/4870/5770 - more texture units (40% more), more shaders (42.5% more), more bandwidth same rops and the performance isn't that much different (I know that many people think that the 5000 series shaders are weaker vs 4000 series shaders, but please bear in mind that 5000 series has better IQ quality and that probably costs that performance amount - read BFG10K articles on AF/AA of 5000 series).
I think less ROPs is what kills 5830 performance.