puppet-foreman does not allow for configuration of PassengerMaxPoolSize
The katello installer puppet code (not the puppet module itself but the foreman module) does not allow for configuring PassengerMaxPoolSize. It defaults to six, which is not difficult to max out, especially on heavily-used systems.
To replicate this behavior without a true load test, alter the 'index' method in /usr/share/foreman/app/controllers/api/v2/users_controller.rb to be:
def index sleep(20) @users = resource_scope_for_index end
After this, you should be able to use hammer to get the user list and max out the passenger worker pool (note that this is different than the httpd worker pool).
The default value of six is OK, but it should be spelled out in the conf file that it's six instead of relying on passenger internal defaults, and it should be configurable via installer options.