Using smarty here is really just because it's very easy to convert the existing templates/code and already gives quite a big improvement over the current implementation. As stated above a lot of the conversion can be automated. I even generated the PHP code for the new menu from the old menu templates.
Another framework you might consider for ispCPv2:
http://www.yiiframework.com/
Seems to be considerably faster than Symfony as well.
@Obi_Wan: Do you mean helping with the smarty conversion or helping the ispCP team on v2?
I'll post a preliminary patch soon so everyone who'd like to help can say so here then we could divide the remaining templates. I'm only working on this during some spare time but I'm making good progress. All the main menu points are already converted.