It would be cool if a page in a board displayed 30 topics (currently 20) and a page in a topic displayed 20 posts (currently 15). The efficiency gain from this is unnoticeable (still there are less PHP requests if you do some math). It would be just much easier to read and concentrate on the topic.
The purely matematical side of this is that there are less PHP requests and queries in total when someone perhaps checks a topic with 60 posts. Most simple way to express this would be:
60 / 15 (current posts per page) = 4
and it takes ~20 queries to display a topic with 15 posts in it
this means to display the whole topic it takes 20*4 = 80.
this means if you display 20 posts per page it will take 3*20 = 60 to show the topic
... that is if of course the posts don't contain anything worth querying (they do, one query per one poster too I suppose).