Wanted to just add that I'm following this closely as well. I am researching moving a large vB3.x site to either vB5 or XF. It looks like my metrics are a bit larger than yours, but you're asking fantastic questions. The other alternative we've been considering is moving to Drupal.
I've been away from the vBulletin community for some years and upon returning to see if vB4/5 had yet matured into a stable product capable of running a very large (5MM post, 3000 concurrent user) site, I've found feedback that suggests that vB5 may not be the way to go.