BuddyPress.org

well the content already hits bp_create_excerpt before updating the activity stream (maybe the default length is too long)

you can try hooking on the filters
apply_filters( ‘groups_activity_new_forum_post_content’, $activity_content, $post_id, $post_text, &$topic ),
apply_filters( ‘groups_activity_new_forum_topic_content’, $activity_content, $topic_text, &$topic ),

and trim it down some more