BuddyPress.org

You have to fool around with the “bp_dtheme_ajax_querystring” filter.

Boone provided a small tutorial on this:
https://buddypress.org/community/groups/how-to-and-troubleshooting/forum/topic/how-can-i-set-show-newblogposts-as-default-on-acitivity-stream/?topic_page=2&num=15#post-42623

Modify the code to detect a group and change the per_page variable if you’re on a group.