Starting with OpenSIPS 1.5.0 the internal DB API provides support for prepared statements. First DB module to implement prepared statements is DB_MYSQL module. See more about the concept of prepared statements in MySQL.
A set of tests were conducted in order to determine the real gain of prepared statements. The tests tried to cover various case - combinations between complexity of a query and the amount of data transfered by a query.
A very good overview on Prepared Statements, that may help you in understating the advantages and disadvantages of prepared statements can be found at http://www.mysqlperformanceblog.com/2006/08/02/mysql-prepared-statements/ .
Following the measurement with the new prepared statements a 25 % increase in performance. See the entire set of test as well as the result interpretation.