Hello Fabio,
Transactions launched asynchronously are handled by MII via JMS and EJB.
You may be able to adapt the behavior in such execution, but it may affect directly the system performance.
I suggest you to check section 'Transactions stuck in PENDING state' of MII handbook.
http://wiki.scn.sap.com/wiki/download/attachments/15603/SAP+MII+Handbook.pdf
Please keep in mind that performance tests are necessary if you modify those values.
Hope it helps !
Please let us know the results.
Best regards,
Raphael