Always great to find some good articles on the workings of BizTalk.
Lee Graber has given some outlines and numbers behind some decisions in his post:
Have you ever thought of using a Send Port Group?
Why? – because 1 subscription is evaluated for all Send Ports in the group,
hence reducing the load for the BizTalk Messagebox DB.
Lee mentions that if you have 8+ identical subscriptions(filter expressions) on individual
Send Ports, then consider creating a Send Port Group and adding the Send Ports to
it.
http://blogs.msdn.com/biztalk_core_engine/archive/2004/07/22/191888.aspx
(check out the last paragraph)