There are many factors that affect performance and the specific operation performed.
There are so many variable to compare, it is very difficult to provide you with specific answers, but I can provide the following general information about H.264 encoding. The actual specs of any specific model processor can be found at http://arc.intel.com.
Re: How will the performance of QuickSync based encoding of H.264 scale across CPU models?:
For products that contain Intel Quick Sync Video support, the H.264 encoding performance is affected the most by the Graphics Max Dynamic Frequency. (This is different than Clock Speed or Max Turbo Frequency). In general, encoding performance will scale with this, if you are comparing the same generation of products. Architectures can change from one generation to the next.
Another important factor is Max Memory Bandwidth (including use of multiple memory channels).
Re: Does it scale directly with CPU frequency ?
You asked about H.264 encoding ant the information above focuses just on converting uncompressed video data to elementary H.264 video stream, however when working with video, other factors often affect the performance of the overall task, including file and stream management (muxing audio, adding headers, network management etc.) For these tasks, the CPU frequency can be important.
re: Does it scale any from HD2000 to HD3000 based IGP's?
In general, Intel HD Graphics 3000 Accelerators provide improved processing capabilities over HD2000 that can greatly improve tasks such as scaling video content. Depending on the amount of video processing or scaling your H.264 encoding involves you will see this affect your overall performance.
When describing Quick Sync Video performance, we concentrate on overall usage models, such as converting camcorder content to handheld device and not just one of the aspect. You can find some general information about this usage model here: http://www.intel.com/performance. We have some comparison of current (2nd) generation product to the previous product for the equivalent market.
Hope this helps.