While Ivy Bridge CPUs do use less power than Sandy Bridge CPUs, I have not seen any review which compared the mobile versions of those CPUs. Again, the 20w difference mentioned above applies to the desktop Ivy Bridge vs. desktop Sandy Bridge CPUs in general.
Ivy Bridge CPUs are about 5% more powerful than a Sandy Bridge CPU so a mobile IB CPU running at 2.0GHz would be equivalent to a mobile SB CPU running at 2.1GHz. Not much of a difference, but Intel never stated Ivy Bridge was going to be more powerful. The main difference is the integrated graphic core. The new Intel HD 4000 is roughly 40% more powerful than the old Intel HD 3000. That's basically going from something as powerful as a desktop Radeon HD 5450 to a desktop Radeon HD 5550.
The Intel HD 4000's performance actually comes somewhat close to the performance of the graphic core in the Llano A8 series; the Radeon HD 6620G. However, the Intel HD 4000 cannot beat the Radeon HD 6620G in any of the benchmarks I've seen. The Intel HD 4000 is more powerful than the nVidia GT 610m graphic card though. Of 10 games benchmarked, the Intel HD 4000 beat the nVidia GT 610m in 7 of them.