Here it is. I haven't tested all possible combinations and configurations since I only need one very simple thing.
There are two files. One contains a couple of patches for GroupSummary bugs (with links to the bug reports where you can find more information) and the other is a slightly modified version of the "experimental" version posted by mystix (i think) and buried somewhere in the 44 pages of this post, which works fine with ExtJS 3.3.0 and group summary (I just fixed a couple of problems I found and the groupsummary bugs that were present in the gridsummary as well).
Oh, and I added and option to choose whether to place the summary row above or below the grid scroller.