standard-jboss.xml file is used only for EJB2.x beans. It's not used for EJB3. To set the activation-config-property through xml file, check this tutorial http://docs.jboss.org/ejb3/docs/tutorial/1.0.7/html/Message_Driven_Beans_with_deployment_descriptor.html
also you could try to modify PoolClass annotation for domain 'Message Driven Bean' in /deploy/ejb3-interceptors-aop.xml (default is maxSize=15).
this should do it for all of your MDBs.
Thx both Jaikiran and Robert.Will give it a try and tell you how it went.