Would be happy to help those who are contemplating a massive migration of an Elgg installation to BuddyPress + bbPress. We just did this and learned lots. The site we converted was large...40K VERY active users. 1.9GB database with massive messaging and activity migration from Elgg to BuddyPress and bbPress. All the hosting is done at WPEngine.com. We went through bumps with them, but all is good now. We had to make major performance tweaks to BuddyPress and bbPress to get performance to a professional level. Let's just say our lead architect "wanted to take a shower every time he looked at the Elgg data model and table structure!" BuddyPress and bbPress was FAR more stable and understandable -- thanks to the amazing WordPress community.