Sorry to re-animate an old discussion, but I have some news on both fronts.
First, the latest versions of Talk include a LOT of performance improvements. Without a back-end, conversations with 5-10 participants should generally be possible (provided enough bandwidth and compute on the clients) . Talk 9 will include more improvements as well as a grid view to make calls with more than 5-6 people also nicer in the user interface.
Second, we continued the conversations with our partner, Struktur, and we are today happy to announce that they have agreed to open source the back-end.