The restriction to airflow volume from a pusher and a puller are different b/c a pusher fan causes airflow turbulance when the air hits the fins/tubes under pressure , whereas a puller does not..there is a big difference in airflow patterns between the two.
The problem with a puller is it has to have a shroud b/c it will attemp to pull supply air from outside the fans circumference, circumventing core flow...but the shroud makes the air supply travel threw the core.