Afaik Evergreen was never starved for shaders. 5870 and 5850 have basically the same perf clock-to-clock. 5830 sucked because it had only 16 ROPs, so it barely beat a 5770 with 40% more shaders. It was easy for Barts to increase the efficiency by increasing the ROP count compared to the amount of shaders. It's basically an OC'd 5830 with double the ROPs. That's the secret to Barts increased perf per mm^2. Now with Cayman the same doesn't apply, so it's up to the shader architecture changes to give similar increases in performance.