We've moved to Invision because, TBH, we've just outgrown phpBB in it's current state. IPB gives us a ton more functionality, and we couldn't wait for the new phpBB (although migration might have been easier!)
Although it is a lot more performant thus far (the new server helps!), the most important thing was stability, security, and I guess a bit of accountability - and this is what a paid licence gave us.
In addition, there was a bit of money in the MoDaCo coffers which meant we could buy IPB, Blogs, Galleries, Copyright Removal and Top Secret Product X which will REALLY take us to the next level.
Think of this as a platform for the future, a solid foundation for what is coming next