XML and Multiple datasources

I am currently converting an existing application which hits a single db
to work with multiple databases. 

The model I am looking at is
datastore>>xml-dbms>>xml-instance>>XSLT>>xml-instance>>application/or
central database>>application.

My concern is speed of transaction, would it be better simply to call
stored proceedures in my central db to hand of queries to target dbs and
translate the resultset for my application to process?

Any suggestions for a more efficient method?

thanks