I created a GroupingCollection2 with better performance (in comparision with GroupingCollection) by tweaking the implementation. You can find a sample/swc here
http://flexpearls.blogspot.com/2008/06/groupingcollection-with-some- better.html If you have some free time please test it and let me know if you find any bugs ! Of-course if you like the performance and would like to use it in your app, feel free :)
