If boosting is meant to be a solution to the back catalogue problem, then it's a horrible way to do it. You'd have to go through and boost every single post from before the hosting instance was followed, and then it'd only show up the user page of the guy who went to all of that effort? (or, realistically, bot).

If what I'm saying is accurate (and I'm still not sure because this is admittedly a bit too complicated for me) then it doesn't sound very useful since individual profiles aren't nearly as important in a forum context when compared to something like twitter, and especially when you can just upvote something and have that show on your profile. Unless I'm mistaken and anything you've upvoted doesn't propagate to another automatically instance while boosts do... but I don't think that's a big enough distinction to have two different buttons? You could just have an upvote also do that.