HI Steffi,
Mobile Server is not required to be deployed seperately. Services for the same come along with the installation of BO server ( full installation )
As Mobile itself does not require any servers of its own, there is little or no effect on performance as such. As all processing would be done by the BO servers.
As Ashutosh said, the MobileBIService.war file is the essence of mobile. If that is deployed under tomcat/webapps, you are done!
Regards,
Atul B