Forum performance inside of BuddyPress is improved in 1.2.7 and will be further improved in 1.3 (which still includes the bbPress stand-alone version.)
Having BuddyPress load all of its code and process everything that it does is the bottleneck in your circumstance. You’d be better off loading bbPress standalone properly themed, then the bbPress plugin, and lastly inside of BuddyPress groups (regardless of standalone/plugin.)
But, I think it’s a horse a piece.
The more you need to load is the slower it’s going to be, and the more important caching becomes. The bbPress plugin will respond to W3TC much better than bbPress standalone and BuddyPress will, since it uses the existing WordPress object cache.
