I just had a look at the ticket but was unable to comment on it, so I will post my comment here instead.
The idea of using the ?p= links is not ideal as a redirect will have to be performed for each link, also for SEO it is far better to have the correct links on the page instead of links that rely on redirect.
My suggestion for posts activity items would be: instead of storing a URL / permalink for each posts activity, why not just store the post ID? – that way you can let the wordpress native function (post_permalink) generate the permalink and no matter how many times you change the permalink settings buddypress activity will instantly link to the correct URLs.
