Going to smcore.org and after a rapid check, it seems you are planing to use some Symfony
component if not the entiere Sf2 framework, isn't it? At which point is Symfony going to be involved?
I think it's a good choice. I know PHPBB4 will also be built around Symfony 2, as well as Drupal 8 with some components.
So good luck, I'm myself working on a Sf2 website with a SMF2 forum ^^